A picnic bill of ₹7,200 was to be shared equally among a group of friends. Three of them did not turn up, so the average amount paid by each friend who did come was ₹80 more than planned. How many friends were in the original group?

With $n$ friends planned, $7200/(n-3)-7200/n=80$. Multiplying out gives $7200\times 3=80n(n-3)$, so $n^2-3n-270=0$ and $n=18$ (the other root is negative). Check: 7200/18 = ₹400 planned and 7200/15 = ₹480 actually paid, a rise of ₹80. 15 is the number who actually came, not the original group. 21 adds the three absentees instead of subtracting them, which would make each share cheaper, not dearer. 90 is 7200/80, dividing the bill by the rise - a figure with no meaning here.