Page 167 - ABCTE Study Guide_Neat
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Answer
The correct answer is C. Forty-one is a prime number; the other numbers are composite. Be careful with
51—it comes from 3 x 17.
Cool Rules
Divisibility rules are tools to help you do less division. The divisibility rules can help you determine if a
number is prime or composite as well as help you break composite numbers into their prime factors, the
topic we will cover next.
Divisibility rules are shortcuts to tell you if one number is divisible by another number. The rules cover
division by numbers 2–10 and allow you to check numbers up through 100. Click on the numbers in the
chart.
Divisibility Rules
2 3 4
5 6 7
8 9 10
Question
Which of the following numbers is not divisible by 3?
A 4,873
B 7,521
C 12,642
D 25,308
Answer
The correct answer is A. The fast way to do this problem is to use the divisibility rule; add the digits of
each number looking for the one that is not a multiple of three. The slow way would be to do each division
problem—even on a calculator, this method will take longer!
Prime Factorization: T's and Tree
This is another topic you might not have thought about since middle school. The Fundamental Theorem of
Arithmetic states that every composite number can be broken down into a unique product of prime
numbers.
