When estimating with compatible numbers, you look for numbers that are close to the original values but easier to work with. For 231, you could round it to 200 to simplify calculations. So, when using compatible numbers, 231 would be estimated to 200.
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The factors of 231 are: 1, 3, 7, 11, 21, 33, 77 and 231
231 written in binary numbers is 11100111
To determine factors of a number it is necessary to divide the number by prime numbers of increasing value. If there is no remainder then the prime number is a factor. The square root of 231 is approximately 15 so it is necessary to divide by all the prime numbers up to 13 (15 is not a prime number). 231 ÷ 2 leaves a remainder of 1 231 ÷ 3 divides exactly 77 times (which is 7 x 11 - both are prime numbers) 231 ÷ 5 leaves a remainder of 1 231 ÷ 7 divides exactly 33 times (which is 3 x 11 - both are prime numbers) 231 ÷ 11 divides exactly 21 times (which is 3 x 7 - both are prime numbers) 231 ÷ 13 leaves a remainder of 10 If we multiply together the three numbers that are shown to be factors 3 x 7 x 11 = 231 this confirms that there are no other factors of 231 except 1 and 231 itself. This last condition is true for every positive integer. 231 is divisible by 1, 3, 7, 11 and 231
3 x 7 x 11 = 231
6 - 123, 132, 213, 231, 312, and 321.