If you take 50 and multiply it by two, you get 100. But, in order to get the right number, you have to multiply 45 by two as well. 45 x 2 is 90. 90 out of 100 is 90 percent. So 90% is the answer.
90% because to find 100% in this fraction you will have to multiply to bottom to get one hundred thus having to do the same to the top getting 90 out of one hundred.