=============================================
Explanation:
S = sum of the first four values
S/4 = 70 since we divide the sum by 4 to get the mean of the first four values
S = 280 is the result after multiplying both sides by 4
Let x be the fifth unknown value. Adding it onto S leads to S+x = 280+x. This represents the sum of all five values.
Dividing that sum 280+x over 5 leads to the new mean 80, which leads to...
----------
(280+x)/5 = 80
280+x = 5*80
x+280 = 400
x = 400-280
x = 120
The fifth number is 120
