6 programming assignments worth 100 points each count for 60% of the grade; i got a total of 595 points

one midterm that counts 20% worth 100 points, and i got an 89;

then the final worth 20% worth 100 points. What grade do i need in the final to keep the A?

my simple approach to solve was this...

60% of 600 points is 360; 324 points is 90%

20% of 100 points is 20; 18 points is a 90%

so my faulted logic says

357 + 17.8 + x = 360

but this equation is wrong, where the hell is my error???

gotta keep studying and can't figure this simple math problme out....