Written methods for addition and subtraction to 1000

Estimate, calculate, check

Every written calculation at Stage 3 should be three steps, not one. Estimate ($398+247$ is about $400+250=650$), calculate, then check the answer against the estimate. A calculation that lands far from the estimate is wrong even if every digit looks neat.

Carrying and exchanging

Carrying is exchanging ten of something for one of the next thing: ten units for one ten, ten tens for one hundred. Subtraction runs the exchange backwards. Write the exchange down — doing it in your head is where the marks go.

Adding in a friendly order

Mentally, $37+48$ is easier as $37+50-2$. Rounding one number to a friendly one and correcting afterwards is a real method, not a shortcut, and Checkpoint accepts it.