Question

The average of 5 numbers is 64. When a sixth number is added, the average becomes 66. What is the sixth number.

Answer 1

( x+y+z+w+k ) / 5 = 64, and so ( x+y+z+w+k ) = 5(64) = 320.

Then, adding a sixth number, a, we get:

[ ( x+y+z+w+k ) + a ]/6 = 66, and ( x+y+z+w+k ) + a = 6(66) = 396.

We now know that ( x+y+z+w+k ) + a = 6(66) = 396, but we also know that
( x+y+z+w+k ) = 320. Thus, 320 + a = 396. Then a = 76.

Answer 2

5 x 64 = 320

66 = (320+x)/6

66*6 = (320 +x)

396 = 320 +x

x = 396-320 = 76

the 6th number is 76