We are simply going to use direct proportion.
The smaller cake is 2/3 the larger cake: let it be = (2/3)S. Let S be size of large cake
I would assume that the bottom tier refers to the larger cake.
Therefore, the bottom tier requires 3/5 cup of water.
Therefore, large cake size S requires (3/5) cup of water,
That means, small cake size, (2/3)S would require....
= 2/3 * 3/5
= 2/5 cup of water.
Therefore, Marcus is correct.