I'm no big brained expert, but here's my take on it. For example; 3/8 + 2/3. The top (or first) number is the Numerator, the bottom (or second) number is the Denominator. The two Denominators have to be the same number. To make the 2 Denominators the same number find the lowest common number they multiply into, in this case it's 24 (3 times 8 = 24 and 8 times 3 = 24) so the new common Denominator is 24 which replaces the 8 and 3 in the original calculation. Now you have multiplied the Denominators you have to do the same with the Numerators. You have to multiply both the Numerators with the same numbers you used to multiply the corresponding Denominators to make the new common Denominator, you had to multiply the first Denominator by 3 to make 24 so you now multiply the Numerator (3) also by 3 and you had to multiply the second Denominator by 8 to make 24 so again you have to now multiply the second Numerator (2) by 8. All this now gives you a new calculation of; 9/24 + 16/24 = 9 + 16 = 25 /24 or 1 1/24