The average (arithmetic mean) of 5,10,15 and x is 20..what is the x ? how do i solve this question?

To find the mean (average), u add up all the numbers and divide by how many numbers there are.

(5 + 10 + 15 + x) / 4 = 20simplify
(30 + x) / 4 = 20...multiply both sides by 4, eliminating the 4 on the left
30 + x = 20 * 4
30 + x = 80...subtract 30 from both sides
x = 80 - 30
x = 50

now lets check it...
(5 + 10 + 15 + 50) / 4 = 20
80/4 = 20
20 = 20 (correct)

so ur answer is : x = 50 <===