A plumber charges $50 to visit a house plus $40 for every
hour of work. Define a variable and write an expression to represent
the total cost of hiring a plumber.

Step-by-step explanation:

Regardless of how many hours the plumber works, you have to pay $50 for him to even walk into your house.  That's non-negotiable.  If he charges $40 per hour, then the amount you pay him obviously depends upon the number of hours he works.  The variable is x and it represents the number of hours the plumber works.  The cost function for hiring this plumber is

y = 40x + 50

That means that $50 is paid even if the plumber does no work at all.  If he does an hour's worth of work, x will be replaced by 1, and the fee he will charge is 40 + 50 = 90.  If he works 2 hours, x will be replaced by 2, the fee he will charge is 80 + 50 = 130.  Etc.  Above is the cost function you need to answer this question.