A student first reduced a certain number by 20% and then increased the resulting number by 25%. If the original number was 1.643 x 1012, then the value of the new number (after reduction and increment) is   1.561 x 1012   1.587 x 1012   1.60 x 1012   1.62 x 1012   1.643 x 1012   Hint Half-n-half Clue