Let us assume the unknown number to be = x
Then
(x/3) = (5x/6) - 15
(x/3) - (5x/6) = -15
(2x - 5x)/6 = -15
-(3x/6) = -15
We can now remove the negative sign from both sides as it will be the same.
(x/2) = 15
x = 15 * 2
= 30.
So the value of the unknown number represented in this equation is 30.
Only thing that is important in respect to this problem is
understanding the words of the problem, otherwise this is an easy problem.