Evie wants groups of 3 notebooks and 5 pens. She already has 12 notebooks. How many pens will she need?

Mar 27, 2018

20 pens

Explanation:

She has four times the amount of notebooks as 1 group would have (1 group=3 notebooks, 5 pens). That means she also needs four times the amount of pens.

$5 \cdot 4 = 20$

Another way to solve this would be to set up a proportion.

$\frac{3}{5} = \frac{12}{p} \rightarrow$ Number of notebooks over number of pens, $p$ represents the number of pens. The ratio of notebooks to pens (or the other way around) stays the same no matter how many groups there are (there are still 5 pens for every 3 notebooks).

$3 p = 60 \rightarrow$ Cross multiply (12 multiplied by 5, p multiplied by 3)

$p = 20$