exam Maths Puzzle Numerical Ability

My mind has gone blank....any help is greatly appreciated:

A boat has a speed of 5 mph in still water. The boat can travel 21 miles with the current in the same time in which it can travel 9 miles against the current. Find the rate of the current.

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exam Other Question

Ten workers could do the job in 12 days. If 8 workers worked for 5 days and then half of them quit, how long would it take for the rest to complete the job? At the kickoff of a football game the receiver catches the football at the left side of the goal line and runs for a touchdown diagonally across the field. How many yards would he run? ( A football field is 100 yards long and 160 yards wide.)