Shaun needs to make mortar for brick-laying. It is made in the ratio 7:3 of sand to cement.

If 210 kg of sand is used, how much cement is needed?

Sand to cement

7:3

210:x

ask yourself what number is needed to multiply by 7 to get 210 (so you'd divide 210 by 7 which is 30). Multiply the other side by thirty as well to get the value of x

3*30 which is 90