I use my calculator but i can tell u a trick i learned
When a question says OF then you know to multiply that so in this instance its 7.5 OF 20% now take out OF and replace it with multiplicantion sign now you have 7.5 x 20% so now you can find out what 20% is 0.20 so now plug it all in 7.5 x 0.20= 1.5
I hope this helps!