Let Emily’s age be E
Since Emily is half John’s age, then John's is: 2E
Four years ago, Emily was E - 4
In 4 years' time. John will be 2E + 4
We then get: 2E + 4 = 4(E - 4)
2E + 4 = 4E - 16
2E - 4E = - 16 - 4
- 2E = - 20
Emily, or
Do you think you can now find John's age?