Carlos works as a tutor for an hour and as a waiter for an hour. This month, he worked a combined total of hours at his two jobs. Let be the number of hours Carlos worked as a tutor this month. Write an expression for the combined total dollar amount he earned this month.

D = a*t + b*(T-t)  

Explanation:

In the example given, four variables are to be considered here,

How many dollars he makes per hour , same as how many dollars he makes as a waiter,  How many hours he worked both as tutor and a waiter  

 Let define our variables

let a = the amount of dollars he makes per hour as a tutor

Let b = the amount of dollars he makes as a waiter

Let  t = number of hours he worked as a tutor

Let T = the total  hours combined  he worked both jobs in a month

Therefore, T-t =  number of hours left he worked as a waiter.      

Then, D =  total dollars Frank received  this month in which is:    D = a*t + b*(T-t)  

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explanation: