End of a prime
I made a list of every number that is the units digit of at least one prime number. How many digits appear in the list?
Age
14 to 16
Challenge level
Problem
I made a list of every number that is the units digit of at least one prime number.
How many digits appear in the list?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
Student Solutions
Answer: 6 digits
Primes: $2,3,5,7,11$ so you can get $1,2,3,5,7$ at the end
Missing: $0,4,6,8,9$ - numbers ending in $0,4,6,8$ are divisible by $2$,
but $9$ is possible e.g. $19$
Therefore $6$ digits appear on the list: $1, 2, 3, 5, 7$ and $9$.