4. A batsman has a certain average of runs for 11 innings. In the 12 th inning he scores 129 runs, thereby increasing his average by 9 runs. What is his average for the first 11 innings?
Solution:
let average of 11 innings b x
then
player got x for 11 times
and 129 for one time
so average is
(11x+129)/12 which is greter than x by 9
=>(11x+129)/12 =x+9
=>11x+129=12x+108
=>x=21


I DIDN'T UNDERSTAND THE SOLUTION.