Answer:
1/18.
Explanation:
Firstly 1 - 1/3 = 2/3 left
Now 1/2 of 2/3 is taken to leave 2/3 - 2/3 * 1/2 = 1/3.
Marsha takes 1/3 of 1/3 = 1/9 so now there is 1/3 = 1/9 = 2/9 left.
So the 4 remaining guests will receive 2/9 * 1/4 = 2/36
= 1/18 of the cake.