"How much should we tip?"
They used the word 'Should', so would 应该 be appropriate? To me the Chinese characters given for the answer would be 'How much do we want to give for a tip'. While they have the same meaning, they are phrased differently.
It is weird that when translating from English to Chinese, the sentence is 'how much should we tip', which is perfectly correct, yet when translating THE VERY SAME sentence from Chinese to English, this phrasing is not accepted.