Mr. Bower has 3 times as many pens as pencils. He has 20 pens and pencils altogether. How many pens does he have?**Solve by trial and error:**

Say you have 1 pencil, then you have 3 pens ( 3 times 1 = 3 )

1 + 3 = 4, so you don't get 20

Say you have 3 pencils, then you have 9 pens ( 3 times 3 = 9 )

3 + 9 = 12, so you don't get 20

Say you have 4 pencils, then you have 12 pens ( 3 times 4 = 12 )

4 + 12 = 16, so you don't get 20

Say you have 5 pencils, then you have 15 pens ( 3 times 5 = 15 )

Since 5 + 15 = 20, then you have 5 pencils and 15 pens.