b represents the number of boxes of pencils Ms. Renaud needs to buy.
From the information given, each box contains 25 pencils. This means that the number of pencils in b boxes is
b * 25 = 25b
She needs 300 pencils for students during testing. If Ms. Renaud currently has 50 pencils and is buying more, it means that the equation that correctly represents b is
25b + 50 = 300
To solve for b, we would subtract 50 from both sides of the equation. We have
25b + 50 - 50 = 300 - 50
25b = 250
Dividing both sides of the equation by 25, we have
25b/25 = 250/25
b = 10