Prime numbers and prime factorisation

In PE class, the teacher wants to arrange the $13$ students into perfect rows and columns for an exercise (for example, $2$ rows of $6$ people).

1. a) Why can they not do this with $13$ students in any way other than a single row of $13$?
2. b) What type of number is $13$ based on its factors?

Classify the following numbers in the table according to whether they are prime (they only have two factors: $1$ and themselves) or composite (they have more than two factors):

$11, 14, 19, 21, 23, 27, 31, 33$

At a card collectors' convention, the display stands are numbered from $10$ to $25$. The organisers want to highlight stands with a prime number using an orange sign. Write the list of all stand numbers that should have an orange sign.

Aoife wants to find out if the number $91$ is prime. To check it, she applies the most common divisibility tests: she notices that it is not even (so it is not divisible by $2$), that its digits add up to $9 + 1 = 10$, which is not a multiple of $3$ (so it is not divisible by $3$), and that it does not end in either $0$ or $5$ (so it is not divisible by $5$). With these data, Aoife concludes in her notebook: "Since it is not divisible by 2, or by 3, or by 5, 91 is a prime number."

1. a) Locate and explain the reasoning error that Aoife made according to the rule for determining whether a number is prime.
2. b) State what check Aoife forgot to carry out before giving her answer and justify why it was essential to do it.
3. c) Correct the exercise: determine whether the number $91$ is prime or composite and, if it is composite, write its prime factorisation.

In a maths class, the teacher says that to find out if a number less than $100$ is prime, it is sufficient to check that it is not divisible by $2, 3, 5$ and $7$.

Explain why there is no need to check if it is divisible by $11$ or by other larger prime numbers. Apply this idea to check if the number $97$ is prime.

To check whether the number $601$ is prime, a student claims that they only need to divide it by the prime numbers less than $25$.

1. a) Justify why the student has chosen the number $25$ as a limit.
2. b) Carry out the necessary divisions following their reasoning and determine whether $601$ is prime or composite.
3. c) What is the largest prime number you would have to test to know if $1009$ is prime?

Look at the following "division tower" (prime factorisation) and identify the missing numbers in the blank spaces so that it is correct:

Find the prime factorisation of the number $40$ using the vertical line method and express the result as a product of powers.

Find out which number it is in each case by working out the powers and multiplying the factors:

1. a) $2^2 \times 3 \times 5 = $
2. b) $2 \times 3^3 = $
3. c) $3^2 \times 11 = $

Find the prime factorisation of the following numbers using the vertical line method. Express the final result using powers where possible:

1. a) $180$
2. b) $252$
3. c) $450$

Fill in the blanks in this factor tree for the number $120$. Remember that each branch must end in a prime number for the factorisation to be complete. Then write the final result using powers.

Sometimes, to factorize numbers ending in zero, we can use mental "shortcuts". For example, we know that $10 = 2 \times 5$, so each trailing zero contributes a factor $2$ and a factor $5$.

Factorize the following numbers using this trick to save steps in the vertical line (you can remove each trailing zero at once by writing $2 \times 5$ on the right):

1. a) $1,200$
2. b) $4,500$
3. c) $18,000$

In the maths lab, an incomplete prime factorisation has been left on the board. Help reconstruct it knowing that the original number is less than $500$.

Calculate the value of the original number $X$, the intermediate quotient $Y$ and the prime factor $P$ knowing that $P$ is the smallest possible prime number that makes $X$ a multiple of $3$.

Find all the factors of the number $N$ knowing that its prime factorisation is $N = 2^2 \times 13$. Is $N$ a composite or prime number? Justify your answer based on the definition of these types of numbers.

Find all the factors of the following numbers using their prime factorisation. Show the process of combining factors:

1. a) $28$
2. b) $42$

Find the smallest number that has exactly $6$ factors, knowing that only the first two prime numbers appear in its prime factorisation. Explain your reasoning by analysing the exponents in its factorisation.

A natural number $N$ has the following property: its prime factorisation is of the form $2^a \times 3^b$. We know that if we multiply $N$ by $2$, its number of factors increases by $3$, and if we multiply it by $3$, its number of factors increases by $4$.

Calculate the value of the exponents $a$ and $b$, and find the number $N$.