Selling price of a cellphone is increased by 20%. For this sale is decreased by 30%. Find the effect of total sales?

Let cost of each cell phone = Rs.100 & Sale = 100 phones.

Money receipt = Rs.(100 x 100) = Rs.10,000

New cost per cellphone = Rs.120 and New sale = 70 phones

New Money Receipt = Rs.(70 x 120) = Rs.8400

Then the effect of sales = decrease in money receipt = 

$\frac{1600}{10000} x 100 = 16\%.$