15/X = 15/ 1.5X+ 1

15/X= 10/X + 1

15/X- 10/X = 1

5/X= 1

X=5

If x in 15/ 1.5X is on the molecule, then

15/X = 15X/ 1.5+ 1

15/X= 10X + 1

Multiply both sides by x at the same time

15= 10X？ +X

10X？ +X- 15=0

Just find x according to the root formula.