For every ton of ore there is 20% copper. We therefore need to calculate what will render 10 tons of copper.
Let x = the unknown amount of ore we need.
20% means 20 over 100 and we require an answer of 10
`therefore 20/100 times x = 10`
`therefore x= 10 times 100/20`
The amount of ore required to obtain 10 tons of copper at a rate of 20% per ton of ore is 50 tons.