We are multiplying 5/8 by something to get an equivalent fraction. What times 5 equals 10. 2 right? So we multiply the numerator by 2 over 2. This means because we multiplied the numerator by 2 we have to multiply the denominator by 2 as well. 8 x 2=16 so our answer is 10/16 without being simplified. If you need to simplify the answer you would factor out the numbers and divide by 2 over 2 which gets you to 5/8.