By selling 20 pens for ₹100 a shopkeeper loses 10%. How many pens must he sell for ₹100 to gain 20%?

Selling 20 pens for ₹100 at a 10% loss means they cost 100/0.90 = ₹111.11, i.e. ₹5.5556 a pen. For a 20% gain each pen must fetch $5.5556\times 1.20$ = ₹6.6667, so ₹100 covers 100/6.6667 = 15 pens. (Shortcut: the number sold for a fixed ₹100 is inversely proportional to the price per pen, so $20\times 0.90/1.20=15.$)