In order to find the missing number, you can write the given situation in an algebraic way, as follow:
(5 + x)/(11 + x) = 2/3
where x is the missing number. To solve the previous equation for x, proceed as follow:
(5 + x)/(11 + x) = 2/3 multiply by 3(11 + x) both sides
3(5 + x) = 2(11 + x) apply distributive property both sides
15 + 3x = 22 + 2x subtract 2x and 15 both sides
3x - 2x = 22 - 15 simplify like terms
x = 7
Hence, the missing number is 7