backpack cost = b

Cheap backpack = b - 15

The total of both items equals 75:

b + b - 15 = 75

Solve for

*b*in the equation b + b - 15 = 75

**Step 1: Group the b terms on the left hand side:**

(1 + 1)b = 2b

**Step 2: Form modified equation**

2b - 15 = + 75

**Step 3: Group constants:**

We need to group our constants -15 and 75. To do that, we add 15 to both sides

2b - 15 + 15 = 75 + 15

**Step 4: Cancel 15 on the left side:**

2b = 90

**Step 5: Divide each side of the equation by 2**

2b/2 = 90/2

b = 45

Cheap backpack = 45 - 15 =

**30**

Source

Source