Example of Word problem:

There is a man who is 21 years older than his son.  5 years ago he was four times as old as his son. How older are both now?

Solution:

Step 1. Let x = Son's Age Now

Step 2. Then,

x + 21 = Father's Age Now

x - 5 = Son's Age Five Years Ago

(x + 21) - 5 = Father's Age Five Years Ago

Step 3. 5 years ago the father was four times as old as his son. (x + 21) - 5 = 4(x - 5)

Step 4. (x + 21) - 5      = 4(x - 5)

x + 16   = 4x - 20

x - 4x  = -20 - 16

-3x = -36

x = 12 years

Solving for the other unknowns:

x + 21   = 12 + 21

x + 21   = 33 years

Answers:         Son's Age Now = 12 years

Father's Age Now = 33 years

Step 5. The man is 21 years older than his son.

12 + 21 = 33 years

Five years ago he was four times as old as his son.

33 - 5 = 28 = 4(12 - 5) = 4 x 7

Thus, the answers check.