question about sub-net mask 128.42.0.0/21 where does it come from/

i was studying sub-netting question for my ccent and i got a question to break down 128.42.0.0/21 into 4 equally large sub nets that can least hold 100 host each.

but my question is where did the /21 come from on the 128.42.0.0. its B class type so shouldn't it be a /16