The answer depends on the context.
If considering the 1877 + 11 then a compatible number would be 1880
but
if considering 1877 + 11,000,000,000,000,000 the compatible number is 0.
540
12.5577
it makes it easier when u round
A compatible number close to 12 is 10. It is easier to work with when performing mental calculations, and it rounds down to a nearby, simpler figure. Another option is 15, which rounds up, making it useful for estimating totals or simplifying division.
Rounding, you see the number and just make it look neater or easier to use. Estimating, you guess a number.
540
12.5577
19.8889
it makes it easier when u round
A compatible number close to 12 is 10. It is easier to work with when performing mental calculations, and it rounds down to a nearby, simpler figure. Another option is 15, which rounds up, making it useful for estimating totals or simplifying division.
11
It is 0.7*600 = 420
Rounding, you see the number and just make it look neater or easier to use. Estimating, you guess a number.
It is a compatible number
As a Roman numeral 1877 is MDCCCLXXVII
estimating
Rounding is going to the nearest tens, hundreds, thousands etc., depending on the problem. Compatible numbers are numbers the work well with each other. Both of these are estimating.