Kim uses decals to decorate 5 cars and 2 motorbikes. she uses 2/3 of the decals on the cars and 2/5 of the remaining decals on the motorbike. she has 6 decals left. how many decals does kim use on each car?

Let the number of decals be D. After using decals on the cars she has D-2D/3=D/3 left and after using decals on the bikes she has (1-2/5)D/3=(3/5)D/3=D/5 decals left. So D/5=6 and D=30.
2/3 of D=20 so she uses 20 decals on 5 cars (4 decals for each car). 2/5 of the remaining 10 decals=4 which uses on the bikes (2 on each bike).
Let’s see if it all adds up: 20 + 4 + 6 = 30. OK. 20 is 2/3 of 30. OK. 2/5 of 10 is 4. OK.
And 20 decals shared by 5 cars is 4 per car.