To write 0.424242 as a fraction you have to write 0.424242 as numerator and put 1 as the denominator. Now you multiply numerator and denominator by 10 as long as you get in numerator the whole number.
He already had 100 into the account so we add that, 25x is derived from the fact that x=the amount of months and 25 is the amount per a month he saves.
