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?

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

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