Solution:
Given that;
Where, C is the cost of hiring a plumber and n represents hours worked
Mustafa and Diego both work as plumbers
a) Mustafa charges $150 to make a service call plus $50 per hour,
The equation representing the cost (C) of hiring Mustafa for n hours will be

Hence, the answer is

b) Diego charges $50 to make a service call plus $75 per hour,
The equation representing the cost (C) of hiring Diego for n hours will be

Hence, the answer is

c) Today, they both worked the same number of hours and earned the same amount of money, then,

Hence, they worked for 4 hours