Answer:
The student completed 13 courses worth 3 credits
Step-by-step explanation:
System of Equations
Let's assign the following variables:
x = number of courses worth 3 credits
y = number of courses worth 4 credits
Some student completed 18 courses thus:
x + y = 18 [1]
The student earned a total of 59 credits:
3x + 4y = 59 [2]
From [1]:
y = 18 - x
Substituting in [2]:
3x + 4(18 - x) = 59
Operating:
3x + 72 - 4x = 59
Simplifying:
-x = 59 - 72 = -13
x = 13
The student completed 13 courses worth 3 credits
