antique vintage flash cards, classroom math & measurements quiz question cards

Lot of about 40 old flash cards, quiz questions on measurements for math classes. These are single sided, with answers at the bottom of the cards.

These cards measure about 6" x 4" overall, and are yellowed and worn. We're guessing these are 1930s or 40s?

They're a little yellowed and worn from time and use, which just adds to their charm.