You are going to make a box out of cardboard...?

Question

The box needs to measure 10cm by 5cm by 5cm; 10% will be wasted in the cutting process. How much cardboard do you need??

Answer 1

total surface area of the box
= 2(10*5) + 2(10*5) + 2(5*5)
= 100 + 100 + 50
= 250 cm²

since 10% will be wasted, this is the amount of cardboard you'll need:

250*(110%)
= 250*1.1
= 275 cm²

Answer 2

Surface area = 2(10 x 5) + 2(5 x 5) + 2(10 x 5)
= 250 cm²
Start with Y cm²
10% is wasted so finish with 90% Y = 0.9Y
Thus 0.9Y = 250
Y = 250 / 0.9 = 278
You need 278 cm² to nearest cm²

Answer 3

First answer is pretty close, but I expect the wastage is 10% of what you start with, not of what you end with.

So 90% of amount needed = 250
Divide by 0.9:
100% of amount needed = 277.777....

To check this, start with this amount and subtract 10% of it, you get 250.

Start with 275 and subtract 10%, you get 247.5