Consider that Tom is a 16-year-old boy and he is 4 times older than his brother Jon. Tell us how old Tom would be when he will be twice as old as his brother?

Given that Tom is 16 years old, at this time he is 4 times older than his brother Jon. At present, the age of Jon will be (16/4 =4). Hence the age of his brother is 4 years. Now if Tom is double the age of his brother when Jon is 12 years old which means 12×2 = 24?
When Tom will be 24 years old then the age of his brother Jon will be 12.