The number of students in 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 mins passage of time, the number of students joining the class gets tripled i.e. multiplied by 3

The number of students joining after 40 minutes from now on is $729 \times 3 \times 3 \times 3 \times 3 \times 3 \times 3$ ( from 20 mins back to 40 mins forward)

The cube root of 729 is 9.

Cube root of given number is $9 \times 3 \times 3 = 81$

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