if you want the strictly mathematical approach....turn the sentence into an equation...
A childs toy consisting of a hand held game and a battery costs £21. The Game costs £20 more than the battery, how much does the battery cost?
This means that 2"x" + 20 = 21 (the item is £20 more than the battery....therefore the battery must be half of the total value - 20)
so 2"x"=21-20 (the magic fairy sits on the "=" and inverts functions as they pass over it)
2"x"=£1
"x" = £1 / 2
"x" = 50p