Or search by topic
How many sets of three prime numbers have the property that the product of the three numbers is exactly five times their sum? (The order of the three numbers is not important).
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
Find the smallest positive integer N such that N/2 is a perfect cube, N/3 is a perfect fifth power and N/5 is a perfect seventh power.
If a two digit number has its digits reversed and the smaller of the two numbers is subtracted from the larger, prove the difference can never be prime.
Take any prime number greater than 3 , square it and subtract one. Working on the building blocks will help you to explain what is special about your results.