Mentally add and subtract 10 or 100 to a given number. Mentally add or subtract with up to one exchange. For example, mentally add 80 to 192. Think 8 + 19 = 27, so the answer is 272.

Conclude by leading this investigation:

Rock Monster

by MathPickle

2.NBT.B.8: Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900.