Answer:
102
Explanation:
Let your number be n
The average of 3 ,15 and your number is found by adding the three numbers and dividing by 3
(3+15+n)/3
That is equal to 40
(3+15+n)/3=40
Multiply each side by 3 to get rid of the fraction
(3+15+n)/3 *3 = 40*3
(3+15+n) = 120
Combine like terms
18+n = 120
Subtract 18 from each side
18-18 +n = 120-18
n =102
Your number is 102