GATE CSE 2021 Set 2 Q8: Class strengths from ratios before and after adding 18

The number of students in three classes is in the ratio 3:13:6. If 18 students are added to each class, the ratio changes to 15:35:21.

The total number of students in all the three classes in the beginning was:

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Answer: C (official GATE 2021 final answer key)

Concept used: Write the classes as multiples of one unknown and use the new ratio.

First step: Let the classes be $3k, 13k, 6k$ ; then $\dfrac{3k+18}{13k+18} = \dfrac{15}{35}$ gives $k$ , and the total is $22k$ .

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Source: official GATE 2021 Set 2 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key