These are infinite geometric series. Geometric series are series where the starting term, a1, is multiplied by a constant number r each time.

3, 6, 12, 24, 48
starting term 3, r = 2

sum of infinite geometric series = a/(1-r)

2 + 1 + 1/2 + 1/4 ...
a1 = 2, r = 1/2
2/(1-(1/2)) = 4

5+2+4/5+8/25
a1=5, r=2/5

5/(1-(2/5)) = 5/(3/5) = 25/3

something that's helpful to know is this: when you start with something like 1/x and multiply by 1/x each time... the sum is 1/(x-1)
1/3 + 1/9 + 1/27 + ... = 1/2 , no need to use the formula