Since Taryn earns 122.50 to mow 7 lawns he charges
122.5/7 or $ 17.50 to mow a lawn.
Since Alastair earns 112.50 to mow 9 lawns he charges
112.5/9 or $ 12.50 to mow a lawn.
The number of hours he spends mowing does not enter this part of the question.
Earning of Taryn per hour = 112.5/7.5 or $15.00 per hour
Earning of Alastair per hour = 122.5/5 or $24.50 per hour