Let the number of strawberry sold = x
The number of vanilla = strawberry + 2 = x + 2
The number of chocolate = vanilla + 11 = (x + 2) + 11 = x + 13
So we have that
x + ( x + 2) + ( x + 13) = 42 simplify
3x + 15 = 42 subtract 15 from both sides
3x = 27 divide both sides by 3
x = 9 = strawberry sold