Loose fruits and veg are cheaper

Asda charges 68p a kilo for loose bananas and £1.40 for a pack of ten. Although many customers believed the pack would be cheaper, buying ten loose bananas cost £1.20 – a saving of 20p.

At Sainsbury's, there are eight fruit in a £1.15 pack of 'Fairtrade Basics' bananas. Buying the same number loose would cost 96p – a saving of 19p.

Tesco sells an 'Eat Me' pack of five bananas for £1, while buying loose fruit would be 40p cheaper at 60p.

In Waitrose a pack of six 'Essential Fairtrade' bananas was £1.38, while buying six loose would cost 72p.

A Which? spokesman said: Supermarkets make it impossible to work out whether it's cheaper to buy bagged or loose fruit. We want them to show clear unit pricing – price per weight or item – so you can compare.

"Big four" supermarkets in UK

Asda is generally the cheapest (an average of about 3%), followed by Tesco, then Morrisons and Sainsbury's.

How to pronounce mathematical exponentiation

The expression b² = b·b is called the square of b because the area of a square with side-length b is b².

The expression b³ = b·b·b is called the cube of b, because the volume of a cube with side-length b is b³.

So 3² is pronounced "three squared", and 2³ is "two cubed".

The exponent says how many copies of the base are multiplied together. For example, 3⁵ = 3·3·3·3·3 = 243. The base 3 appears 5 times in the repeated multiplication, because the exponent is 5. Here, 3 is the base, 5 is the exponent, and 243 is the power or, more specifically, the fifth power of 3, 3 raised to the fifth power, or 3 to the power of 5.

The word "raised" is usually omitted, and very often "power" as well, so 3⁵ is typically pronounced "three to the fifth" or "three to the five".