from itertools import groupby

lst = [('ask', 421), ('answer', 634), ('answer', 125), ('answer', 285), ('ask', 755), ('ask', 451), ('answer', 935), ('ask', 633)]

result = []
for k, g in groupby(lst, lambda x: x[0]):
	i = -1 if k == 'ask' else 0
	result.append(list(g)[i])

print(result)