A batsman scores 87 runs in the ${21}^{\mathrm{st}}$ match of his career. His average runs per match increases by 2. What was his average before the ${21}^{\mathrm{st}}$ match.

Let the average runs of batsman in 20 match = x According to question

$(x+2)=\frac{20x+87}{21}$

or $20x+87=21x+42$

x = 45 

So the average runs of batsman before 21 match = 45