Alex's child Drew is exactly one fifth of Alex's age.

In 21 years, Alex will be exactly twice Drew's age.

Blake is exactly seven times the age of their child, Glen.

In 8 years, Blake will be three times the age of Glen.

How old are Drew and Glen now?

Puzzle Copyright © Kevin Stone

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Hint

A little bit of algebra might help.

Answer

Drew is 7 years old and Glen is 4 years old.

The first half of the question involves Drew (D) and Alex (A).

Drew is currently one fifth of Alex's age, so:

(1) A = D x 5

In 21 years, Alex will be twice their age, so:

(2) A + 21 = (D + 21) x 2

Using (1) in (2) gives:

A + 21 = (D + 21) x 2

5D + 21 = (D + 21) x 2

5D + 21 = 2D + 42

3D = 21

D = 7

So Drew is 7 (and Alex is 35).

The second half of the question involves Glen (J) and Blake (B).

Blake is exactly seven times the age of Glen:

(3) B = J x 7

In 8 years, Blake will be three times the age of Glen, so:

(4) B + 8 = (J + 8) x 3

Using (3) in (4) gives:

B + 8 = (J + 8) x 3

7J + 8 = (J + 8) x 3

7J + 8 = 3J + 24

4J = 16

J = 4

So Glen is 4 (and Blake is 28).