Seven Up
The number 27 is special because it is three times the sum of its digits
27 = 3 (2 + 7).Find some two digit numbers that are SEVEN times the sum of their digits (seven-up numbers)?
Are there any three digit 'seven-up' numbers. Is it possible to find a number K such that there is a three digit number that is K times the sum of its digits (a K-up number)?
You may also like
Double Digit
Choose two digits and arrange them to make two double-digit numbers. Now add your double-digit numbers. Now add your single digit numbers. Divide your double-digit answer by your single-digit answer. Try lots of examples. What happens? Can you explain it?
Repeaters
Choose any 3 digits and make a 6 digit number by repeating the 3 digits in the same order (e.g. 594594). Explain why whatever digits you choose the number will always be divisible by 7, 11 and 13.
Big Powers
Three people chose this as a favourite problem. It is the sort of problem that needs thinking time - but once the connection is made it gives access to many similar ideas.