Page 299 - Math Smart - 7

13.2.2 Direct proportion

When the ratio of 2 pairs of quantities are the same, we say the quantities are
proportional to each other.

To cook enough rice for 3 people, you need 2 cups of water and 1 cup of
i
dry rice. How many cups of water and cups of dry rice will you need to serve
rice to 12 people?

If two quantities are in the same ratio, they are said to be in direct proportion.
They increase or decrease at the same rate.
• If one quantity is doubled, then the other will double too.
• If one quantity is tripled, then the other will triple too.
• If one quantity is halved, then the other will be halved too, and so on.

Example 2

Emily is paid $5.30 per hour at work.
How much does she earn for working 4 hours?
Solution
1 h—►$5.30.
4h—►AX $5.30 = $21.20
Emily earns $21.20 for working 4 hours.

A food supplier for a restaraunt packs 300 pieces of tuna puffs in 12 cartons. The
restaurant owner orders 2100 pieces of tuna puffs to be delivered the next day.
How many cartons must the food supplier prepare?

Solution
12 cartons ► 300 puffs

X 7 X 7
? cartons ■>2100 puffs

2100 is 7 times of 300. So, we need 7 times the number of cartons.
12 cartons x 7 = 84 cartons
As the number of cartons
B ► 2100 increases, the number of
12 " ► 300 tuna puffs increases. This
CI]= 2100 X 12 is direct proportion.
300
= 7 X12
= 84
The supplier must prepare 84 cartons to pack 300 pieces of tuna puffs.