Marcin buys 7 rulers and 15 crayons for £7. A ruler costs 12p more than a crayon. Find the cost of one crayon.

To solve this problem we're going to use an equation. First let's write down the information that we do know.Let's use (r) for ruler and (c) for crayon.
So 7 rulers and 15 crayons cost £7 (or 700 pennies) --> 7r + 15c = 700But we also know that r = c +12
So we can substitute c + 12 into the first equation where r is at the moment.
7(c+12) + 15c = 700
Now let's expand the brackets. We multiply 7 by c to get 7c and 7x12 t0 get 84
7c + 84 +15c = 70022c + 84 = 700 Here I'm going to take 84 away from both sides. 22c = 616
Remember this is a non calculator paper so we need to find an easy way of dividing 616 by 22.
616 divided by 22 is 28 so c = 28Each crayon therefore costs 28p