[Grammar] two-thirds of the pancake were

[Oceanlike]

I think I should use "were" instead of "was"

- Shocked that two-thirds of the huge cake were eaten by Lizzy, I ......

(a) Since I can count a cake, so I should use "were" for "two-thirds of the huge cake". Is this right?

(b) If I say "two-thirds of the clean water", then I should use "was"?

 

[GoesStation]

I think I should use "were" instead of "was"

- Shocked that two-thirds of the huge cake were eaten by Lizzy, I ......

(a) Since I can count a cake, so I should use "were" for "two-thirds of the huge cake". Is this right?

No. Fractions less than one are singular.

(b) If I say "two-thirds of the clean water", then I should use "was"?

Yes, for the same reason.