When did you realize you had lost your purse? When I ... money to pay the eat.

Kelas: SMA
Pelajaran: English
Kategori: past tense
Pembahasan:

When did you realize you had lost your purse?
When I was going to use the money to pay the meal (not the eat).

The use of 'was going to' is to state the intention in the past. Since the question was already in past tense (simple and past perfect), so the most possible answer is to use 'was going to'.