1/3(y-15)=40. This step is correct, simplify within the parentheses.
your next step is not correct because EVERYTHING on the left side of the equation is multiplied by 1/3 and you just multiplied 15 by 1/3, so.....
3(1/3(y-15))=3(40). I would multiply both sides of the equation by 3 to eliminate the fraction giving you.....
y-15 =120. Makes it whole numbers and easier to solve
y-15 +15 = 120 +15. Perform the opposite operation to the whole number to isolate the variable
y = 135. Simplify and you get your solution