For #3 - #6, everything is simply just dividing.
#3 3 strikes per inning
#4 9 points per minute
#5 30 beats per 20 seconds = 1.5 beats per second
90 beats per 60 seconds = 1.5 beats per second
The rates are equivalent.
#6 15 pages in 20 minutes = 0.75 pages per minute
10 pages in 15 minutes = 0.666666666667
The rates are NOT equivalent.
For #7, all you have to do is multiply.
#7 40,000 gallons per second
1 minute = 60 seconds
40,000 * 60 = 2,400,000
The dam releases 2,400,000 gallons per minute.