Its simple! All you have to do is:  Convert each price to a price per kilogram. [Or price per litre if the item is a liquid.]  Then choose the cheapest price.  Note! There are several different ways to do the maths. This PowerPoint shows only one of the ways.

Which bag of Easter eggs is the better value? The purple packet costs $9.98 for 475g or $22.59 per kg The red packet costs $3.48 for 180 g or $20.78 per kg $20.78 for 1 kg is less than $22.59 for 1 kg. The red 180g packet is better value for money. The smaller packet is the better value.

A 500g bag of pasta costs $3.65. What is the price per kilogram? 500g g = 1000g Two bags of pasta contain 1000g or 1 kg. $ $3.65 = $7.30 Or 2 × $3.65 = $7.30 The price per kilogram is $7.30

Which packet of rice is the better value? Two 500g packets make 1kg. $2.10 × 2 = $4.20 In the 500g packet the rice costs $4.20 per kg. In the 5 kg pack one kg of rice costs $22.60 ÷ 5 = $4.52 $4.20 is cheaper than $4.52. The 500 g packet is the better value.

A 375 ml can of soft drink is priced at $1.40. Calculate the price per litre. Two cans of drink contain 2 × 375 ml = 750 ml. Three cans contain 3 × 375 ml = 1125 ml Two and a bit cans hold 1000 ml or 1 litre. Two cans is not enough and three cans is too many. Exactly how many cans do I need?

1 L is the same as 1000 mL. I will need cans to make 1 litre. 1L = 1000 ml

How much will 1 litre costs? Cost of 1 litre = cost for 1 can × number of cans Cost of 1 litre = $1.40 per can × cans = $ litre will cost $ mL for $1.40 is equivalent to $3.73 per litre.

What size Sultana Bran is the best value?

In the small packet the price of the Sultana Bran is $44.22 per kg. One small packet weighs 45g ÷ 45 = packets weigh 1 kg One small packet costs $1.99. × $1.99 = $ g for $1.99 is equivalent to $44.22 per kilogram.

In the 500g box, the price of the Sultana Bran is $11.58 per kilogram. One 500g packet of Sultana Bran costs $5.79. Two packets of 500g Sultana Bran make 1 kg. 2 × $5.79 = $ g for $5.79 is equivalent to $11.58 per kilogram.

In the 820g box the price of the Sultana Bran is $9.04 per kilogram. The large 820g box of Sultana Bran costs $ ÷ 820 = boxes of 820g sultana Bran make 1 kg. × $7.41 = $9.04

45g for $1.99 = $44.22 per kg 500g for $5.79 = $11.58 per kg 820g for $7.41 = $9.04 per kg The 820g packet is the best value. The 45g packet is very poor value!

Summary of how to do the maths Calculate the number of containers required to make 1 kilogram or 1 litre. (1000 ÷ size of the container in g or mL) Calculate the price of 1 kg or 1 L by multiplying the price per container by the number of containers required to make 1 kg or 1 L. Remember! There are other ways to do the maths. Can you do the calculations a different way?