Right menu

Featured resource

Five Practices

5-practices

Productive mathematical discussions lead to students who can think, reason and engage effectively in quantitative problem solving, characteristics which are much needed but may not routinely emerge from our classrooms. So just how can teachers create such discussions?

Members: $ 30.00 inc.GST Others: $ 37.50 inc.GST


The potato paradox

You have 100kg of Martian potatoes, which are 99 percent water by weight. You let them dehydrate until they’re 98 percent water. How much do they weigh now?

Logic:

Initially the non-water weight is 1kg, which is 1% of 100kg. After dehydration, 1kg is 2% of how many kilograms? In order that that percentage be twice as big, the total weight must be half as big. Therefore the weight is 50kg.

Algebra:

The weight of water in the fresh potatoes is 99% of 100 kg = 99kg

Let x be the weight of water lost when they dehydrate (i.e. x kg of water is lost)

So the final result is (99 – x) kg water

However, the final result can also be expressed as (100 – x) kg of potatoes which contain 98% water, that is 0.98 (100 – x) kg water

Equating

(99 – x) = 0.98 (100 – x)

99 – x = 98 – 0.98x

1 = 0.02x

x = 50

therefore 50 kg of water is lost and the potatoes weigh 50kg