Hands up: what would you need to work out before you could be sure?

Three comparisons on the board. For each one, watch how we get to the price of one before we decide anything. The first one is a surprise: two very different yoghurt tags.

Next is a lunch deal. One shop sells 4 sandwiches for €6.00. The other sells 6 of the same sandwich for €8.40. We divide each total price by its pack size to find the cost of one sandwich, then compare those two single prices to see which pack is better value.

This next one is sold by weight, so counting items will not help. We line both packs up on the same measure: the price of one 100 g piece. The 1 kg block is ten 100 g pieces, and the 250 g block is two-and-a-half 100 g pieces. Watch which pack costs less for the same 100 g, even though one looks dearer at first.

• Eggs: 6 for €2.10 or 12 for €3.60
• Juice: 1 litre for €1.50 or 2 litres for €2.80
• Pasta: 500 g for €0.90 or 1 kg for €1.70
• Kitchen roll: 4 rolls for €3.20 or 6 rolls for €4.20