There is a large number of cubes in a bag... urgent

Question

There is a large number of cubes in a bag. Jason wants to estimate the total number. He takes 10 random cubes, marks them and puts them back. He shakes the bag then takes out 20 random cubes of which 3 are marked. Work out an estimate for the total no of cubes.

Answer 1

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

 

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

 

total = 10•20/3 = 200/3 ≈ 67