If there are 3 sons multiply the fraction by 3 and then you have 12/15 because 12/15 is equal to 4/5, so the remaining is 3 out of 15, so divide that between the 3 sons and boom, 1/15th per son.
Ok so if you notice every choice is solved for a, all you have to do is isolate "a" in 7a-8b=10x. Start by moving 8b to the other side and to change the sign. Now it is 7a = 10x + 8b. Then divide everything by 7 and you get a = (10x + 8b)/7