Question

a man buys a certain numberof articles at 15 for Rs. 112.50 and sells them at 12 for Rupees 108 find gain

Answer 1

First off Rs.112.50 is for 15 articles. This means C.P of one article is:
$\frac{Rs.112.50}{15}=Rs.7.5$.
Then he sells them at Rs.108 for every dozen. This means S.P of one article is:
$\frac{Rs.108}{12}=Rs.9$

The formula for profit is $S.P.-C.P.$.
We have to just subtract those two numbers now:

$P=S.P.-C.P.$
$P=9-7.5$
$P=1.5$

Ans=Rs. 1.5