Question

David was asked to solve the problem below:

If a = 5x - 4 and b = 3x, what is the value of a - b?
David used the steps below to solve the problem
Step 1: a - b
Step 2: (5x-4) - 3x
Step 3: 5x-4-3x
Step 4: -2x
Determine whether David's solution is correct. If he is correct, show the work that proves he is
correct. If he made an error, explain what he should have done instead, and provide the correct
solution.

Answer 1

• not correct
• Step 4: 2x -4

Explanation:

You want to know if David's solution of -2x for (5x-4) -(3x) is correct, and what David should have done.

Solution

David's solution is correct up to Step 4.

At that point, it appears as though David treated -4 as if it were -4x. The constant cannot be combined with x terms. Step 4 should have been ...

Step 4: (5 -3)x -4 = 2x -4

David's incorrect solution should have been 2x -4.