Hi , there is two possible way to answer the question :
First ,using simple logic :
amount of urea present in 15%(120 g )=18 g
amount of urea present in 20 %(120 g )=24 g
so the difference between two equation is equivalent to amount of urea must be add to have required concentration = 24 -18 = 6 g
but this answer is not present in the choice
Second ,by using alligation method :
100 %------ 5 part of 100 %
----------20%------------
15% ---------80 part of 15 %
therefore,
80 part equivalent to 120 g
5 part equivalent to ? g
so amount of urea required =(5 *120 ) /(80 ) =7.5 g
but I think according to question (WHAT AMOUNT OF UREA SHOULD BE ADDED ) the first answer is correct because the question ask from amount................
if I wrong please correct me.....