simple math help

[OutHouse]

on our road trip last weekend i was trying to explain to my daughter how distace, speed and time work with each other.

i can do it in my head but how can i write it down so she will understand

say we that we are 129 miles away from our destination. How fast will we have to travel to get there in 3.45 hours (207 minutes)

 

[Ricemarine]

37.4 MPH

Distance/Time = Speed
Speed * Time = Distance
Distance/Speed = Time

 

[sygyzy]

129 miles / 3.45 hours = 37.39 mph = ~38mph.

 

[BigJ]

To start off, I would do it in a time interval of less than 60 minutes. This keeps the converting to hours part out of it.

 

[OutHouse]

Originally posted by: sygyzy
129 miles / 3.45 hours = 37.39 mph = ~38mph.

 

[tfinch2]

Originally posted by: sygyzy
129 miles / 3.45 hours = 37.39 mph = ~38mph.

can't think right now

 

[DOSfan]

Originally posted by: tfinch2

Originally posted by: sygyzy
129 miles / 3.45 hours = 37.39 mph = ~38mph.

wrong

129 miles / 207 minutes != 38 mph

there isn't 100 minutes in an hour

It isn't 39mph?

129/207 minutes = 0.6231eic. mpm

0.6231etc * 60 (since there are 60 minutes in an hour) = 37.3913eic mph.

 

[nycxandy]

Originally posted by: tfinch2

Originally posted by: sygyzy
129 miles / 3.45 hours = 37.39 mph = ~38mph.

wrong

129 miles / 207 minutes != 38 mph

there isn't 100 minutes in an hour

What?

 

[mdchesne]

Originally posted by: Citrix

say we that we are 129 miles away from our destination. How fast will we have to travel to get there in 3.45 hours (207 minutes)

129miles /3.45hours = X miles/hour

tell her to think about labels.

 

[Koenigsegg]

Originally posted by: DOSfan

Originally posted by: tfinch2

Originally posted by: sygyzy
129 miles / 3.45 hours = 37.39 mph = ~38mph.

wrong

129 miles / 207 minutes != 38 mph

there isn't 100 minutes in an hour

It isn't 39mph?

129/207 minutes = 0.6231eic. mpm

0.6231etc * 60 (since there are 60 minutes in an hour) = 37.3913eic mph.

37.39 --> 38, not 39.

You got pwned even after the edit.

 

[sygyzy]

I am not sure why people are getting thrown off by the minutes. He gave that information to FOOL ALL OF YOU. Speed is measured in miles per hour. You know how many miles it is. And you know how many hours. Figure out the speed.

 

[OdiN]

Originally posted by: sygyzy
I am not sure why people are getting thrown off by the minutes. He gave that information to FOOL ALL OF YOU. Speed is measured in miles per hour. You know how many miles it is. And you know how many hours. Figure out the speed.

Yeah lol...there are no minutes in the equation.

 

[birdpup]

I was caught badly on this one once as a tutor. I felt like dirt.
I now remember this equation by traveling down a dirt road.

d = r * t

d i rt

 

[JohnCU]

Tell her if x is a function of position, then the derivative of x with respect to time is velocity and the derivative of velocity with respect to time is acceleration.

 

[Stojakapimp]

I dun get what the confusion is. He gave us 129 miles and 3.45 hours. The answer that we want is speed, which is miles/hour. Therefore, divide 129miles/3.45=37.4miles/hour. Shouldn't his be correct?

 

[akubi]

here's something used to teach grade school kids (or the really stupid kids in jr/high) how to use the equation.

draw a triangle like so...
....d.....
./.....`..
r------t

to find one of the three values, you "cover it with your hand" and look at the remaining two corners.

example... 1) you want to find distance when you know the speed and time: cover up the 'd'.... the r and t are right next to each other.... so you multiply them.

2) you want to find the avg speed when you know the distance traveled in t time... cover up the 'r'... the d is on top of t. so divide d by t

3) you want to find the time taken to go distance 'd' at speed 'r'... cover up the 't'... the d is on top of r. so divide d by r

now you still have to remember which corner is labeled what...

 

[IonYou]

Originally posted by: JohnCU
Tell her if x is a function of position, then the derivative of x with respect to time is velocity and the derivative of velocity with respect to time is acceleration.

No no no you have to learn about the curvature of the female form before you can learn about the slope of a curve. That's why they don't teach that stuff to pre-pubescents.

 

[dullard]

How old is the daughter? If she is 10+ she should be able to grasp the very powerful concept of multiplying and dividing units.

Don't think of distance as being d = 129 miles. Think of it as d = 129 * miles. Think of time as being t = 3.45 * hours.

Then think of units that can be multiplied and divided. Suppose you want a final answer of mph. Written out that unit is: miles / hours. See how the units are divided?

Now without knowing any equations at all, you can solve this problem correctly (and ~99% of all math/physics/chemistry problems). Take whatever data you have with the unit of miles and divide that by whatever data you have with the unit of hours:

(129 * miles) / (3.45 * hours) = (129/3.45) * (miles / hours) = 37.4 miles / hour.

See how the units can be moved around using the standard arithmatic rules?

Add one more simple concept and she can solve anything she comes across. What simple concept? You can always multiply by 1 and the answer doesn't change. People have lots of trouble with this concept, so maybe she'll need to be 13+ to understand it. How is it used in this problem? Well, 60 * minutes = 1 * hour. Or rearranging: (60 * minutes) / (1 * hour) = 1. Anytime I want, I can multiply by 1. So anytime I want, I can multiply by (60 * minutes) / (1 * hour) and the answer won't change.

Lets do the problem in minutes. Let t = 207 * minutes and d = 129 * miles. Thus:

d / t = (129 * miles) / (207 * minute) = (129 / 207) * (miles / minute) = 0.623 * miles / minute.

But that isn't the unit I want; I want miles per hour. So lets multiply by 1:

(0.623 * miles / minute) * 1 = (0.623 * miles / minute) * (60 * minute) / (1 * hour) = (0.623 * 60) * (miles*minute/minute/hour) = 37.4 * miles / hour.

Note how the "minute" units cancel, minute / minute = 1. Those two concepts will let your daughter solve just about anything without knowing any formula or ever worrying about units.

 

[Stojakapimp]

how about you just explain how to use units to figure out equations. You know that the answer you want is in units of miles per hour, or rather miles/hour. So how do you get miles/hour...you divide miles, by hours...simple as that.

 

[Legendary]

What dullard said.
I think in high school that's taught under the name of "factor-label" in basic physics.

 

[Kelemvor]

Best way to start is to note that Speed in Mph is really The distance in miles divided by 1 hour.

So 55MPH is really 55Mi / 1 Hr

You can use that to make the rest of the quation.

so 129 Mi / 3.45hr = X Mi / 1Hr

= 37.4 Mph

 

[speg]

How old is she?

Just tell her to match up the units. Speed is in units of distance/time. etc... if you want to go 200km in 2 hour, and you speed is in km/h or 200/2 or 100.