If a boy sells a book for Tk. 450 he gets a loss of 10%, then finds the cost price. To gain 10%, what should be the selling price?

1) Find the cost price

Let C.P. of book = x and S.P. = Tk. 450
S.P. of book = C.P. – (10% of C.P.)
S.P. = x – (0.10x)
450 = 0.9 x
x i.e cost price = Tk. 500

2) Find Selling Price to gain 10%.

Now, we are asked to find a selling price to gain 10% profit.
Selling price = (100 + Gain%)/100 × C.P.
Selling price = (100 + 10)/100 × 500
Selling price = (110/100) × 500
= Tk. 550