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.

An aeroplane flies along the four sides of a square field at speeds of $200,400,600$ and $800\, km / hr$. The average speed of the plane in the flight around the field in $km / hr$ is:

Ashish has earned an average of $4200$ dollars for the first eleven months of the year. If he justifies his staying on in the US on the basis of his ability to earn at least $5000$ dollars per $month$ for the entire $year,$ how much should heearn (in $dollars$) in the last $month$ to achieve his required average for the whole $year$ ?

A cricketer had a certain average of runs for his $64$ innings. In his $65^{th}$ innings, he is bowled out for no score on his part. This brings down his average by $2$ runs. His new average of runs is?

A cricketer, whose bowing average was $12.4$ runs/wicket takes $5$ wickets for $22$ runs in a match, thereby decreases his average by $0.4$ The number of wickets, taken by him before this match was