John has twin sisters. The sum of the ages of John and his twin sisters is 39. In 4 years’ time the twins will be 18. How old will John be in 4 years’ time?

With a problem like this first you must pull the necessary information out of the question. It can make it more familiar if we assign single letters to the unknowns. In this case the unknowns are, Johns age (what we are trying to find) and his sisters age.Lets say John's age = J and his sisters age = S. From the first sentence as sum means add together we find: J + 2S = 39 :: We use two S as John has two sister, We can rearrange this by subtracting 2S to find an equation to tell us Johns age: J = 39 - 2S Now we need to use the second sentence to form an equation to find the sisters ages.As their current age plus four years is 18.S + 4 = 18 ::We only use a single S as the text only referrer to a single sister S = 18 - 4 :: Write out all your workings even for simple steps to allow the examiner to see what you are doing S = 14 so each sister is 14 years old.We can now plug S = 14 back into our first equation to find J = 39 - 2(14) :: Remember to multiply by 2. Use brackets to make workings clear to read J = 11. So we have found Johns age to be 11 years old.However we are not done yet, as the question asks for Johns age in 4 years time so we must add for to the add calculated: 11 + 4 = 15So John will be 15 years old.