Question

A parent rewards a child with 50 cents for each correctly solved mathematics problem and fines the child 30 cents for each incorrectly solved problem. If the child nets $22.00 after 100 problems. How many problems were solved correctly?

Answer 1

65

Explanation:

Let c = number of correct answer.

Let i = number of incorrect answers.

There are 100 questions, so

c + i = 100

The correct answers are worth 0.5c.

The incorrect answers are worth -0.3i (a negative value because 30 cents is taken away for each incorrect answer)

The total worth of the answers is 0.5c - 0.3i. The total worth of the answers is $22.00, so the second equation is

0.5c - 0.3i = 22

We have a system of two equations in 2 unknowns:

c + i = 100

0.5c - 0.3i = 22

We'll use the substitution method to solve the system of equations.

Solve the firt equation for c:

c = 100 - i

Now substitute 100 - i for c in the second equation.

0.5(100 - i) - 0.3i = 22

50 - 0.5i - 0.3i = 22

-0.8i = -28

i = 35

c = 100 - i

c = 100 - 35

c = 65

Answer: 65 problems were solved correctly.