to calculate the percentage of a number simply multiply the number by the percentage. To do this you first need to convert the percentage to a decimal by moving the decimal point two places to the left and removing the percent sign.
15%=.15
18%=.18
After converting the percentages to decimal numbers, multiply them by the number for which you are trying to determine the percentage.
.15 times 33 is 4.95
.18 times 90 is 16.2