Together, sarah's two cats have the same mass as her dog, 11 kg. the mass of one cat is
1500 g greater than the mass of the other. what are the masses of the two cats?

First, lets do all calculations in kg. 1kg=1000 grams
Lets set 1 cat's mass as x.
Lets set the other cat's mass as y.
1) x+y=11
2) y=x+1.5   

Lets plug eq 2 into eq 1.
x+x+1.5=11
2x+1.5=11
2x=9.5
x=4.75 kg

If one cat weighs 4.75 kg, then the other must weigh 6.25 kg.

So cat1=x
cat2=y
x+y=11kg
11kg=11000g
x=y+1500
subsitute
y+1500+y=11000
subtract 1500
2y=9500
divide by 2
y=4750
subsitue
x=4750+1500
x=6250

one cat weighs 6250g and the other weighs 4750g