x=weight of whole crab
y=weight of half crab
If a crab and a half weigh a pound and a half:
x+y=3/2
but the half crab weighs half as much again as the whole crab:
y=3x/2
==> x+3x/2=3/2 or x=3/5
what do half the whole crab and the whole of the half crab weigh?
"the whole of the half crab" is ambiguous, i.e., either all of the half crab or double the half crab. So here are solutions for both.
all of the half crab:
x/2+y=x/2+3x/2=2x=6/5=1.2 lb
double the half crab:
x/2+2y=x/2+3x=7x/2=21/10=2.1 lb