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There is a large number of cubes in a bag.
Jason wants to work out an estimate for the number of cubes in the bag.
He takes at random 10 cubes from the bag.
He puts a mark on each cube and then puts each cube back in the bag.
Jason shakes the bag and then takes at random 20 of the cubes.
There is a mark on 3 of the cubes.
Work out an estimate for the total number of cubes in the bag

Answer :

marked(10)/total = 3/20 (from sample)

total/10 = 20/3 (inverting the above proportion)

total = 10•20/3 = 200/3 ≈ 67

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