**13. The cost of bananas is increased by Re. 1 per dozen, one can get 2 dozen less for Rs. 840. Find the original cost of one dozen of banana.**

**Sol.**Let the cost of banana per dozen be Rs. x.

^{840}/

_{x}

^{840}/

_{x+1}

^{}

^{840}/

_{x}–

^{840}/

_{x + 1}= 2

^{1}/

_{x}–

^{1}/

_{x+1}] = 2

^{x+1- x}/

_{x(x+1)}] = 2

^{1}/

_{ (x}

^{2}

_{+ x)}] = 2

^{2}+ x)

**+ 2x – 840 = 0**

^{2}^{2}+ x – 420 = 0

^{2}– 20x + 21x – 420 = 0