Question 611003
To simplify the ratio, divide both values by their greatest common divisor, also referred to as gcd. It happens the gcd of 66 and 55 is 11. So the ratio can be rewritten as follows:

6*11  :  5*11

Dividing both of those values by 11 results in the following form where each term is relatively prime (the gcd of both values is 1).

6 boys  :  5 girls