Varun borrowed some money from karan at 12% p.a simple interest. He then added an amount and lent the total to charan at 18% p.a. simple interest. At the end the year, Varun gained rs672. If Varun lent rs8000 to charan, then how much did varun borrow from karan?

Let Varun borrow rs X from karan. so, interest @12%SI at the end of one year is 1.12X. Hence he has paid 0.12X extra
Now let he added rs. Y to X and gave to charan which is (X+Y). so,interest @ 18%SI at the end of one year is 1.18(X+Y). hence he received 0.18(X+Y) extra.
overall he gained 0.18(X+Y)-0.12X = 672------equation(1)
and it is given that he gave 8000 to charan
so X+Y=8000------equation(2)
solving (1) and (2) we get
X=6400 and Y=1600
hence varun borrowed rs.6400 from karan

How did this become 1.12x and 1.18x . Please explain