Let us consider a, b, c, d, e, f the six numbers.
If the average of 6 numbers is 19 then
(a + b + c + d + e + f)/6 = 19
=> a + b + c + d + e + f = 114 (1)
And if the average of 5 numbers is 20 then
(a + b + c + d + e)/5 = 20
=> a + b + c + d + e = 100 (2)
We will replace (2) in (1) And it will result that
100 + f = 114 => f=114, the sixth number.
