A cricketer gives $12.4$ runs per wicket. He gives $26$ runs and takes $5$ wicket in a match after which his average becomes $12$ per wicket. How many wickets had been taken till the last match?

What was the average in $Rs.$ daily expenditure of a man in $1999$ who spent $Rs. \,6535$ in the first half year and $Rs.\, 88165$ in the last?

Average rainfall on Monday, Tuesday, Wednesday and Thursday is $420.5\, cm$ and average on Tuesday, Wednesday, Thursday and Friday is $440.5\, cm .$ If the ratio of rainfall for Monday and Friday is $20: 21,$ find the rainfall in $cm$ on Monday and Friday.

The average of nine numbers is $50.$ The average of the first five numbers is $54$ and that of the last three numbers is $52 .$ Then the sixth number is?

The average of runs scored by a cricketer in his $99$ innings is $99.$ How many runs will he have to score in his $100^{\text {th }}$ innings so that his average of runs in $100$ innings may be $100 ?$

A student finds the average of ten $2 -$digit numbers. While copying numbers, by mistake, he writes one number with its digits interchaged. As a result his answer is $1.8$ less than the correct answer. The diference of the digits of the number, in which he made mistake, is