The number of students in a class 20 minutes ago was 729. If the number of students joining the class triples every 10 minutes. What is the cube root of the number of students joining after 40 minutes from now?

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Solution

The correct option is A 81
For every 10 minutes passage of time, the number of students joining the class gets tripled i.e. multiplied by 3.
From 20 mins back to 40 mins forward, we have total of 60 minutes. So, the students get tripled six times after every 10 minutes.

The number of students joining after 40 minutes from now $= 729 \times 3 \times 3 \times 3 \times 3 \times 3 \times 3$

The cube root of 729 is 9.

The cube root of the new number of students $= 9 \times 3 \times 3 = 81$

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