STEP - BY - STEP EXPLANATION
What to find?
The number each instructor teach per week.
Given:
Total number they teach per week =39
let e = number of classes Erica teaches per week and b = the number of classes Bo teaches per week.
e =2b - 15
Step 1
Form the linear equation.

Step 2
Substitute e=2b-15 into the above.

Step 3
Collect like term.


Step 4
Divide both-side of the equation by 3.


Step 5
Determine Erica's age.

ANSWER
c. 18 Bo; 21 Erica