We have to use rounding or compatible numbers to estimate the difference of 823 and 242.
Since, a quick way to estimate the difference between two numbers is to round each number and then subtract the rounded numbers.
Therefore, rounding the number 823 to the nearest tens = 820
Rounding the number 242 to the nearest tens = 240
The difference between the estimated numbers
= 820 - 240
= 580
Therefore, the estimated difference is 580.