A cricketer whose bowling average is 12.4 runs per wicket takes 5 wickets for 26 runs and thereby decreased his average by 0.4. The number of wickets taken by him the last match was-

Let the number of wickets taken till the last match be x
$$\eqalign{ & \Rightarrow \frac{{12.4x + 26}}{{x + 5}} = 12 \cr & \Rightarrow 12.4x + 26 = 12x + 60 \cr & \Rightarrow 0.4x = 34 \cr & \Rightarrow x = \frac{{34}}{{0.4}} \cr & \Rightarrow x = \frac{{340}}{4} \cr & \Rightarrow x = 85 \cr} $$