Here's a 2-variable solution:

PRESENTLY
Jack is now 14 years older than Bill.
Let B = Bill's present age
Let J = Jack's present age
So, we can write: J - B = 14

10 YEARS IN THE FUTURE
In 10 years, the both of them are 10 years older. So...
Let B + 10 = Bill's age in 10 years
So, J + 10 = Jack's age in 10 years

In 10 years Jack will be twice as old as Bill
So, to make their future ages EQUAL, we'll need to double Bill's age.
That is: (Jack's age in 10 years) = 2(Bill's age in 10 years)
So, we get: J + 10 = 2(B + 10)
Simplify: J + 10 = 2B + 20
Rearrange to get: J - 2B = 10

So, we now have 2 equations with 2 variables:
J - B = 14
J - 2B = 10

Subtract the bottom equation from the top equation to get: B = 4

So, Bill is PRESENTLY 4 years old.
This means that Jack is PRESENTLY 18 years old

How old will Jack be in 5 years?
Jack will be 23 years old

Answer: D

Aside: Looks like the 1-variable solution is probably faster.

Cheers,
Brent

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