average is the sum divideided by the number of numbers added together
so avarage of 3 number is 15 means
three numbers represented as x+y+z so
[tex] \frac{x+y+z}{3} [/tex] = 15
(x+y+z)/3=15
multiply both sides by 3 to clear fraction
x+y+z=45
45 is the sum of the 3 numbesr
double of that is
45 times 2=90
