Two pens and a pencil cost Rs 25; a pen and two pencils cost Rs 20. One pen costs

Let a pen cost $p$ and a pencil cost $q$. Then $2p+q=25$ and $p+2q=20$. Double the first equation and subtract the second: $(4p+2q)-(p+2q)=50-20$, giving $3p=30$ and $p=10$. A pen costs Rs 10 (option B).