well, the answer quite simple ....there are 4! ways to arrange these numbers...and as 4! = 4*3*2*1 = 24
hence 24 is the correct answer....
u can also remember it by theorem of multiplication as.....in first place (I.e. first code can be any no. out of 4,5,2&7 ....so 4*.....
as first place is acquired by a certain no. that leaves three no. to fill third place and when third place I'd occupied it leaves 2 numbers to fill second place and lastly only one no. to fill the last place .....so it's result will be like 4*3*2*1.
I know this is pretty much confusing ....but still I tried my best....
if anything troubles u here feel free to ask me