A man sold 30 articles for Rs. 100 and gained 20%. The number of articles he bought for Rs. 100 was:

Selling Price (SP) of 30 articles =Rs. 100.
Gain =20%.

Let the Cost Price (CP) of 30 articles be X.

SP=1.20×X
100=1.20×X
X=
1.20
100
​
=
12
1000
​
=Rs. 
3
250
​

So, the CP of 30 articles is Rs. 
3
250
​
.

Now, we need to find how many articles can be bought for Rs. 100:

Number of articles=
(
3
250
​
)
30
​
×100=
250
30×3
​
×100=
250
90
​
×100=
10
90×4
​
=36