A person ordered 5 pairs of black socks and some pairs of brown socks. The price of a black pair was thrice that of a brown pair while preparing the bill the bill clerk inter-changed the number of black and brown pairs by mistake which increased the bill by 100% what was the number of pair of brown socks in the original order?

A. 10
B. 15
C. 20
D. 25
Explanation:
Let the bought x pairs of brown socks and the price of each brown pair be Y.
Then total cost = 5x3Y+xy
Changed cost = 5xY+x*3Y
According to the question .
5y+3xy –(15y+xy)/15y+xy x 100% = 100%
=> 5y+3xy-15y-xy/ 15y+xy x100% = 100%
=> 5y+3xy = 15y+xy+15y+xy
=> 5y+3xy = 30y+2xy
=> xy= 25y
=> x=25.
Hence the original pair of brown socks = 25.