Why do fractions need to have a common denominator before you add or subtract them? ary]\t

Acurious child asked his mother: “mommy, why are some of your hairs turning grey? ” the mother tried to use this occasion to teach her child: “it is because of you, dear. every bad action of yours will turn one of my hairs grey! ” the child replied innocently: “now i know why grandmother has only grey hairs on her head.”