To ensure he never eats the whole cake, bobby only decides to eat half of what’s left for each serving. if 2/3 of the cake are left initially, and no one else but bobby eats the cake, how much of the cake will bobby have eaten by his 5th serving?

1st serving: 2/3 * 1/2 = 2/6 or 1/3
2nd serving: 1/3 * 1/2 = 1/6
3rd serving: 1/6 * 1/2 = 1/12
4th serving: 1/12 * 1/2 = 1/24
5th serving: 1/24 * 1/2 = 1/48

1/3 + 1/6 + 1/12 + 1/24 + 1/48 
1/3 * 16/16 = 16/48
1/6 * 8/8 = 8/48
1/12 * 4/4 = 4/48
1/24 * 2/2 = 2/48
1/48 * 1/1 = 1/48

16/48 + 8/48 + 4/48 + 2/48 + 1/48 = 31/48

Bobby ate 31/48 of the cake in total.