A pupil does the following calculation in his head:
1121 + 788 − 210 − (−592) and says that the answer is 1291.

Explain why this answer must be incorrect.
Rounding each of the numbers gives the following approximation:

1000 + 800 − 200 − (−600) = 2200
The estimation gives an answer of 2200. As 1291 is just over half this amount it is likely to be incorrect.