7.2 Solving ratio problems where the total of one part of the ratio is given
Take a look at the worked example below:
You are growing tomatoes. The instructions on the tomato feed say:
- Use 1 part feed to 4 parts water
If you use 600 ml of water, how much tomato feed should you use?
These questions make much more sense if you look at them visually:
You can now see clearly that 600 ml of water is worth 4 parts of the ratio. To find one part of the ratio you need to do:
600 ml ÷ 4 = 150 ml
Since the feed is only 1 part, feed must be 150 ml. If feed was more than one part you would multiply 150 ml by the number of parts.
Activity 14: Ratio problems with one part given
Practise your skills by tackling the ratio problems below:
A recipe requires flour and butter to be used in the ratio 3:5. The amount of butter used is 700 g. How much flour will be needed?
When looking after children aged between 7 and 10, the ratio of adults to children must be 1:8.
a.For a group of 32 children, how many adults must there be?
b.If there was one more child in the group, how would this affect the number of adults required?
To find one part you do 700 g ÷ 5 = 140 g
To find the amount of flour needed you then do 140 g × 3 = 420 g flour.
To find one part you do 32 ÷ 8 = 4.
Since adults are only 1 part, you need 4 adults.
(b) If there were 33 children, one part would be 33 ÷ 8 = 4.125.
Since you cannot have 4.125 adults, you need to round up to 5 adults so you would need one more adult for 33 children.