Twelve problems that subtract a multiple of 10 from another, like 70 − 30, 50 − 20 and 90 − 40. A reminder at the top shows the trick: 50 − 20 is really 5 tens take away 2 tens, which leaves 3 tens, or 30.
This is the place value side of first grade subtraction. A child who already knows 5 − 2 finds they also know 50 − 20, and that is a big moment for number sense.
Problems like 40 − 40 are there to check that zero tens makes sense too. It is a short page and a good one to follow up with base ten blocks if a child needs to see it.