One third of a number is 15 less than five sixths of a number

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.

The answer is option c.

She will need 5/8 bags for each square yard.