Three consecutive odd numbers
$3, 5, 7$ are a set of three consecutive odd numbers which are all prime.
$109, 111$ and $113$ are a set of three consecutive odd numbers which are not all prime $(111=37\times3)$.
Can you find another set of three consecutive odd numbers which are all prime?
If not, might it be impossible?
Mathematicians aren't usually satisfied with testing a few examples to convince themselves that something is always true, and look to proofs to provide rigorous and convincing arguments and justifications.
Can you prove that there is only one set of three consecutive odd numbers which are all prime?
Below is a proof that has been scrambled up.
Can you rearrange it into its original order?
Extension:
Take a look at Take Three from Five which requires similar reasoning to this problem.
Why do this problem?
This problem, along with the rest of the problems in the Proof for All (st)ages feature, provides an excellent context for observing, conjecturing and thinking about proof, and for appreciating the power of algebra.
Possible approach
These printable cards for sorting may be useful:
Three Consecutive Odd Numbers Proof Sort
Possible support
Encourage students to work in pairs on the proof sorting exercise.