To solve this, put the number of boys into a fraction out of a whole, then Convert to a percentage; there are 11 boys out of 20 total students (11+9) so you could write a fraction 11/20. To become a percentage, it must be a fraction out of 100. To do this, take the denominator and find the Least Common Multiple of itself and 100, or you can find a number that multiplies by the denominator to get 100 (either way should get the same answer)... LCM is 100, so multiply denominator, 20, by 5 to get 100, the. Multiply the numerator by 5 (anything done to the top must be done to the bottom) and you get 55/100. Percentages are fractions out of 100, so the answer is the numerator: 55%.
Alternatively, you can set up a proportion of the fraction to a number out of 100.
9/20=x/100
Cross multiply to get 20x=900, then Divide to get 55
