finding the fifth number

Question

the average of 5 numbers is 13. four of the numbers are 11,9,15 and 26. what is the fifth number?

 

Answer 1

the underlying formula that you want to use to solve this one is

(11 + 9 + 15 + 26 + x) / 5 = 13

solving that algebraically for the value of "x" yields

61 + x = 13(5)

61 + x = 65

x = 4

meaning that the missing number here is 4

Answer 2

The four numbers add up to 61. The fifth number is x. For the average of the five numbers to be 13,
   (61+x)/5 = 13     so 61+x = 65 (13 times 5)  and  x = 4 (65-61).