Find x if the average of 20, 20, 19, 13, and x is 15

(20+20+19+13+x)/5=15
add in parenthesis and multiply 5 to both sides
(72+x)/5 [×5]=15 [×5]
(72+x)=75
subtract 72 from both sides
x=3

1. isolate the variable, in this case, d. subtract 35.5 on both sides of the equation.2. 52 subtracted by 35.5 is 16.53. therefore your answer would be d= 16.5this is the basics of pre algebra, isolating variable through adding, subtracting, multiplying, and dividing, if you have any other questions to ask just comment!