Answers:
John = 12 years old
Mike = 36 years old
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Step-by-step explanation:
j = John's current age
3j = Mike's current age, since he's three times as old as John.
Now fast forward the clock 12 years to get these future ages
j+12 = John's future age
3j+12 = Mike's future age
We're told that at this point in the future, Mike will be twice as old as John, so,
Mike's future age = 2*(John's future age)
3j+12 = 2*(j+12)
3j+12 = 2j+24
3j-2j = 24-12
j = 12
Therefore, John is currently 12 years old.
This makes Mike to be 3j = 3*12 = 36 years old currently.
Twelve years into the future John and Mike will be 12+12 = 24 and 36+12 = 48 respectively. Notice that 24*2 = 48 to help confirm the answers.