Arthur D. answered 03/14/17
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Mathematics Tutor With a Master's Degree In Mathematics
If two numbers are both divisible by 7, then their sum or difference is divisible by 7. (this works for all numbers)
23,331 and 4291 are both divisible by 7
their difference is therefore divisible by 7
23,331-4291=19,040
7(3333)-7(613)
7(3333-613)=7(2720)=19,040
there are other ways also but this is the simplest
you could factor 19,040 but that involves division
19,040=2*2*2*2*2*17*7
there is a rule also...
1) subtract the ones digit
2) divide by 10
3) subtract two times the original ones digit
repeat as many times as you need to get a number that you recognize as divisible by 7
19,040-0
19,040
19,040/10=1904
1904-0=1904
repeat
1904-4=1900
1900/10=190
190-8=182
repeat
182-2=180
180/10=18
18-4=14 and now you recognize that 14 is divisible by 7
again, the first method is the easiest

Arthur D.
tutor
You're welcome, Sam.
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03/14/17
Sam F.
03/14/17