Two mobile phones are sold for ₹9900 each — one at a 10% gain and the other at a 10% loss. What is the overall loss percent on the whole deal?

When two items sell at the same price with equal gain and loss of $x$%, there is always a loss of $x^2/100$ %. Here $10^2/100=1$%. Check: CPs are 9000 and 11000, total 20000; SP total 19800; loss 200 = 1%.