The constant of proportionality (k) for the amount Jeff earns mowing lawns, calculated by dividing his earnings by the hours worked, is $7.50 per hour.
Jeff earns money by mowing lawns and his earnings are directly proportional to the hours worked, which is a concept in linear equations with a constant of proportionality. To determine this constant (k), we divide his earnings by the number of hours worked. Jeff earned $37.50 for 5 hours of work, so k = $37.50 ÷ 5 hours = $7.50/hour. This means that for every hour Jeff works, he earns $7.50.
The correct answer to the question is option a) $7.50/hour, which is the constant rate of money he earns per hour of work.