by selling 400 articles , a man lost SP of 30 articles . had he purchased them for Rs 537. 50 , what would have been the SP of 20 articles .

By selling 400 articles , a man lost S.P of 30 articles .
This means C.P. of 400 articles - S.P. of 400 articles = loss = S.P. of 30 articles
$\Rightarrow$ C.P. of 400 articles = S.P. of 430 articles
Now, the C.P. of 400 articles is Rs.537.50.
So, S.P. of 430 articles = Rs.537.50
Then, S.P. of 20 articles = $\frac{537.50}{430}\times 20$ = 25

Therefore, selling price of 20 articles is Rs.25