quadratic equation

let the number of toys is x

so his cost per tpy=180/x

he sold total x-1 toys at the rate of (180/x)+1

his toy was free to him, so it's as if he bought x-1 toys and sold them for 190 Rs.

S.P. of  x-1 toys=(x-1)[(180/x)+1]

so

(x-1)[[(180/x)+1]=190

solving we get

 x=20