okay so the two equations are (b-book d-dvd)
d-4=b
and 3d+b=28
so to solve this if we know that a dvd costs £4 more than a book and we have 3 of them then if we take 3 lots of 4 away from the 28 then we will have an amount equal to 4 books
so 28-12=14
14/4= £3.50
if 1 book is £3.50 then a dvd is £7.50 as it is £4 more
