Average of 50 numbers was calculated as 20 when three numbers, 26, 36 and 64 were wrongly read as 31, 46 and 59 respectively. What is the correct average?

Average formula = $$\frac{Sum\ of\ all\ observations}{Total\ number\ of\ all\ observations}$$

As given the average of 50 numbers is 20 ,i.e. sum of total numbers is $$20\times {50}$$ = 1000.

Sum of wrongly taken numbers = 31+46+59=136

Sum of correct numbers = 26+36+64 = 126

Hence, the actual change in the observation will be 126-136=(-10)

Now the actual sum of all observations is 1000-10=990

New actual average will be now = $$\frac{990}{50}$$ = 19.8.