Remark
Drawing that diagram was a very good idea. If you can picture something happening, you are half way there. I tend to use a table, but the diagram will come to the same thing.
Olivia Dawn
Start 176 80
Donate x x Subtract
What's left 176 - x 80 - x
Equation
176 - x = 5*(80 - x) Olivia had 5 times as many books as Dawn did. Remove the brackets.
Solution
176 - x = 400 - 5x Add 5x to both sides
176 + 5x - x = 400 Combine terms on the left
176 + 4x = 400 Subtract 176 from both sides
4x = 400 - 176 Combine terms on the right.
4x = 224 Divide by 4
x = 224/4
x = 56 Answer