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.