If 4/10 of the population have blood type O+, that means 40% of the population have that blood type. To change a fraction to percent, the easiest way is to make the denominator 100. To do this, multiply the whole fraction by 1 (1/1, 2/2, etc). In this case, multiply by 10/10. 4/10 * 10/10 is equal to 40/100. Then, if you have a denominator of 100, the numerator is the percent. Therefore, 40/100 is 40%.