A friend of ours was explaining the healthy smoothie that she makes for her breakfast every morning. Her recipe was not really to our tastes, but she explained it to us in great detail.

The recipe she uses is based on fruit juice, and she rotates through orange juice, apple juice and pineapple juice on a three-day cycle. Apparently, it is important to include different fruits in your diet, not just the same one each day.

She then adds some fruit, and as with the juices, she keeps a variety in her freezer and takes out the following morning's fruit every night before she retires to bed, to allow it to defrost. She did not give us details of the cycle of fruits that she uses, and none of us asked.

Finally she adds porridge oats for energy and roughage.

She makes up her smoothie in a 16 ounce cup using a one fluid ounce measuring cup to put in the correct ratio of ingredients.

But then it started to get interesting, as she told us how much her breakfast smoothies cost. The fruit juices are all the same price, and work out at 36p per fluid ounce. The frozen fruits are also all the same price, and work out at 45p per fluid ounce cup. And the porridge oats cost 43p for each fluid ounce cup.

She informed us that her smoothie was extremely healthy and worked out at only 39p per fluid ounce.

How many cups of juice, fruit and oats does she use?

At 39p per ounce her smoothie must cost a total of £6.24. The only way to arrive at that figure using whole ounces of the ingredients is ten ounces of juice, and three ounces each of fruit and porridge oats.

10 x 36 + 3 x (43 + 45) = 624