Question 34935
OK long way first:

235 325 523 723
237 327 527 725
253 352 532 732
257 357 537 735
273 372 572 752
275 375 573 753


there are no other 3 digit numbers made up from 2,3,5 or 7.


So... "quick" method:
Think of the number like ABC... how many numbers could be placed in position A? Answer any of the 4.
OK, so we have a number in position A. Whatever it is, we now have 3 numbers left to put into position B, so altogether we have 4*3 possibilities.
And finally we have 2 numbers to put into position C, so altogether we have 4*3*2 permutations --> 24.


Count the number of examples i wrote at the start: there are 24 different 3 digit numbers.


jon