Quantitative Aptitude :: Chain Rule - Discussion

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Explanation:
food is there for 54 days for 2000 students,
But after 15 days, remaining food was sufficient for 2000 students for 39 days only ( as 54-15 = 39 )
Let x number of students came in hostel after 15 days,
=> Total number of students after 15 days = 2000 + x
=> Remaining food was enough for (2000 + x) students for 20 days

Note: (u) means up arrow and (d) means down arrow.

students days
------------------
200(u) 20(u)
(200+x) 39

=> 200/(200/x)= 20/39
=> 2000*39 = (2000 + x) * 20
=> 3900 = 2000 + x-10
=> x = 1900