2 kids had the same number of cars. After she gave him 72 cars, he had 3 times as many cars as her. How many stickers did they have in all?

At the start 'she' has x cars and 'he' has x
cars.

If 'she' has x cars then she has x-72 cars when she
gives them to 'him'.

Now 'he' has three times as many cars
as 'she', so he has 3(x-72) cars.

But since they both had x
cars to begin with, this must be equal to x+72 after 'she' has given him 72
cars.

So now we have an
equation

3(x-72)=x+72

Distribute

3x-216=x+72

2x=216+72    
subtract x from both sides, and add 216 to both
sides

2x=288     divide both sides by
two

x=144   So they started out with 144
cars

So they both had 144 cars this means they started out
with 288 cars.