Question 933084
A man bought N toys for Rs180.
<pre>
Each toy cost him Rs{{{180/N}}}
</pre>
He kept 1 for his own use and sold the other N-1 toys for Rs{{{(180/N+1)}}}. 
<pre>
Selling N-1 toys for Rs{{{(180/N+1)}}} he received Rs{{{(N-1)*(expr(180/N)+1)}}}
</pre>
he made a profit of Rs10, and he paid Rs180, So he received Rs180+Rs10 = Rs190.
<pre>
So   

Rs{{{(N-1)*(expr(180/N)+1)}}}{{{""=""}}}Rs{{{190}}}
{{{(N-1)*(180/N+1))}}}{{{""=""}}}{{{190}}}  
{{{(N)(180/N)+(N)(1)+(-1)(180/N)+(-1)(1)}}}{{{""=""}}}{{{190}}}
{{{(cross(N))(180/cross(N))+N-180/N-1}}}{{{""=""}}}{{{190}}}
{{{180+N-180/N-1}}}{{{""=""}}}{{{190}}}
{{{179+N-180/N}}}{{{""=""}}}{{{190}}}
Multiply all terms by N
{{{179N+N^2-180}}}{{{""=""}}}{{{190N}}}
This is a quadratic equation, get 0 on the right:
{{{-11N+N^2-180}}}{{{""=""}}}{{{0}}}
Write terms in descending order:
{{{N^2-11N-180}}}{{{""=""}}}{{{0}}}
Factor:
{{{(N+9)(N-20)}}}{{{""=""}}}{{{0}}}
{{{N+9=0}}}, {{{N-20=0}}}
{{{cross(N=-9)}}}, {{{N=20}}}

He intially bought 20 toys.

Edwin</pre>