JEE Main 2020 · MathsMultiple choiceSingle correctMediumMulti-step

JEE Main 9 January 2020, Shift 1, Maths Q59

Question 59 of 75 in this shift, Maths question 9 of 25, Section A.

A spherical iron ball of 10 cm radius is coated with a layer of ice of uniform thickness that melts at a rate of

50\,\mathrm{cm}^3/\mathrm{min}

. When the thickness of ice is 5 cm, then the rate (in cm/min.) at which of the thickness of ice decreases, is :

(1) $\frac{1}{36\pi}$
(2) $\frac{5}{6\pi}$
(3) $\frac{1}{18\pi}$ Official answer
(4) $\frac{1}{54\pi}$

Official answer

Option 3

NTA final key (Jan 2020).

Chapter: Application of Derivatives
Topic: Chain Rule
Idea tested: Chain Rule in Related Rates

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Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.