Multiple choice

The volume of the frustum of a cone is 7,740 cm$^3$. The bottom and top radius are 14 cm and 7 cm. Find its height. (Use $\pi$ = 22/7). Round off your answer to nearest whole number.

  1. 21 cm

  2. 22 cm

  3. 23 cm

  4. 24 cm
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Volume of frustum = (1/3) * pi * h * (R^2 + r^2 + R*r). 7740 = (1/3) * (22/7) * h * (14^2 + 7^2 + 14*7). 7740 = (1/3) * (22/7) * h * (196 + 49 + 98) = (1/3) * (22/7) * h * 343. 7740 = 22 * h * (343/21) = 22 * h * 16.333. 7740 = 359.33 * h. h = 21.54, which rounds to 22.

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