In order to find a percentage of a number, the easiest thing to do is convert it to a decimal. You would do this by thinking of what part of one your percentage represents, which, in this case, is 0.20 of one. Then, you would multiply this by your number given:
When you add 0.30 to 1.50, your answer will be 1.80.
I hope this helped you out! Feel free to ask me any additional questions if you have any. :-)