Your student will be learning:
Example of compatible numbers:
279 + 128 is about 275 + 125.
These numbers add nicely: 75 + 25 = 100 and 200 + 100 = 300.
Finally, 100 + 300 = 400. So, an estimate for 279 + 128 is 400.
Resources
Your child will round numbers to the nearest 10 or 100 against the clock.