Answer:
Take your question 6/7 + 4/8, and find a common denominator. 7 and 8 both go in to 56, so set both denominators to 56.
The question now reads 6/56 + 4/56.
Multiply the numerator by the number of times the denominator was multiplied to get to the common denominator. 8 goes into 56 7 times, and 7 goes into 56 8 times.
Therefor:
7 x 8 = 56, and 8 x 7 = 56.
Now multiply the numerator by the amount of times the denominator went into the common denominator.
8 x 6 = 48 and 7 x 4 = 28.
So 48/56 and 28/56.
We could then simplify, by dividing both sides by the same number.
12/14
7/14
Then add the numerators only.
That would give us 19/14
Hope that helps, :D.