2. A strawberry milkshake is made in the ratio of 2 parts
strawberry ice cream to 7 parts of milk.
If we fill our blender with 3 scoops of ice cream,
then how many scoops of milk should
we add to the blender?
Ice Cream : Milk
2 : 7
3 : m
Image Source: Google Images
Where m = scoops of Milk (Continued next slide)
3. We can solve this question using a method called
“Cross Multiplying” like this:
Ice Cream : Milk
2 : 7
3 : m
2xm=3x7
Image Source: Google Images
2m = 21
2 2
m = 10.5 scoops of milk
4. - Write the Ratios out Vertically
- Make sure there are three numbers present
and one unknown letter variable
- Cross Multiply the Items
- Divide both sides by the number which is in front
of the letter variable
- Write the simplified answer
5. a : 15 = 2 : 5 Find the value of “a”
2 : 5 Write the ratios out vertically,
and then Cross Multiply
a : 15
2 x 15 = 5 x a (Note writing 2 x 15 = a x 5 is also okay)
30 = 5a “5” is the number in front of the
letter variable and so divide
30 = 5a both sides of the equals sign
5 5 by 5 and simplify.
6=a a=6
6. 4 : 11 = 80 : k Find the value of “k”
4 : 11 Write the ratios out vertically,
and then Cross Multiply
80 : k
4 x k = 11 x 80 (Note writing k x 4 = 80 x 11 is also okay)
4k = 880 “4” is the number in front of the
letter variable and so divide
4k = 880 both sides of the equals sign
4 4 by 4 and simplify.
k = 220
7. The instructions for mixing two stroke lawn mower fuel
say to mix 1 part Oil to 24 parts Petrol or Gasoline fuel.
If we have a 200ml of Oil, then how much Petrol do
we need to add to make two stroke lawn mower fuel ?
Note that the usual method for making
the fuel is to first pour the oil into a 5
litre fuel can, and then add the required
amount of petrol in stages, giving the can
a good shake, with the lid on, to mix the
fuel thoroughly at the end of each stage.
(Continued next slide)
Image Source: http://www.hobuk.co.uk
8. Oil : Petrol = 1 : 24 = 200 : P where “P” = Petrol Amount
1 : 24 Write the ratios out vertically,
and then Cross Multiply
200 : P
1 x P = 24 x 200
1P = 4800 “1” is the number in front of the
letter variable and so divide
1P = 4800 both sides of the equals sign
1 1 by 1 and simplify, or do 1P = P
P = 4800ml or 4.8 litres (So we just fill the 5 litre can with Petrol)
9. Famous Australian artist Ken Done has created the following designs
for a new Australian flag.
The ratio for the flag size is Length : Width = 5 : 3
Tiffany is going to make one of these flags for her Graphic Design
Project, and plans for the length of the flag to be 90 cm long.
What width should she make the flag ?
(Continued next slide) Image Source: http://www.ausflag.com.au
10. Length : Width = 5 : 3 = 90 : w where “w” = Width of Flag
5 : 3 Write the ratios out vertically,
and then Cross Multiply
90 : w
5 x w = 3 x 90
5w = 270 “5” is the number in front of the
letter variable and so divide
5w = 270 both sides of the equals sign
5 5 by 5 and simplify.
w = 54 cm
