Kelvin's supply of labor over this wage range is 3.80
Explanation:
This is Price elasticity supply issue, and wages / hours are independent variables.
Price elasticity of supply tests the response to the supply of a product or service following a shift in its market price. The supply of a good will increase as its prices rise according to basic economic principles. On the other hand, when its price declines, goodwill supplies decline
So, PES ( Kevin's labor) = {(10-7)/[(10+7)/2)} /{(50-40)/[(50+40)/2]}
= 24.75/6.5 = 3.80