@ATIF FAUN

Dear Brother,

The formula to calculate average is:

Average = Sum of Items/Total number of items

Now, in this case, we don't know how many runs the batsman has scored in first 11 innings so we suppose that the average runs of the batsman in first 11 innings is 'x'. Now use the above formula to calculate the total sum of runs the batsman made in 11 innings:

Put Total number of items = 11, Average = x, so Sum of items(innings) = 11x

Now, it is told that batsman made 129 runs in 12th innings. So to find average of 12 innings we add the runs mande in 12 innings and divide it by 12.

Average of 12 Innings = Sum of runs in 12 innings/ 12(Total no. of innings)

Now, runs in 11 innings are 11x and for 12th innings runs are 129. Sor total runs for 12 innings are: 11x+129
Divide it by 12 to find average of 12 innings: (11x+129)/12

Now the question says that this new average(average of 12 innings) increase his old average(average of 11 innings) by 9 runs.

So old average in this case was 'x'. So according to question:

New Average = Old Average + 9
(11x+129)/12 = x + 9
Solving this equation yields x = 21.

It means that batsman's average for 11 innings was 21.


Hope it helps.
Regards,

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