Let cost price of one article be x.
So cost price of 15 articles will be 15*x=15x.
Cost price of 24 articles=selling price of 15 articles.
So selling price of 15 articles= 24*x=24x.
Cost price of 15 articles=15x.
Gain= 24x-15x= 9x.
So gain%= gain/CP ×100.
= 9x/15x *100 .
= 3/5 ×100 .
= 3×20=60%