a number of for digit is someting who is wrote

abcd where those for letter are any decimal digit (0-9) you want

(when a=0 or ab=00 this may not be considered as four digit number is up to you to check or to prcise in your answer the choice you make)

so...

first lets form numbers whith four diferent digits

you chose a youve got 10 (or 9 if you consider the remark the contrary of below) then you choose b you have now 9 possibility then c 8 possibility then d 7 possybility left then youve got one number whith differents digits

i know because i have done this sort of exercise a lot of time (but i admit i have no formal explanation to give you)

that they are then 10*9*8*7 numbers of four different digits that you can form

try whith two digit whit less digits ( 0 and 1) ect and get your own explanation!!

then try to find the proportion of numbers between oo and 99 that satisfy the odd even relation you mentioned

as they are as many numbers between 3500 and 3600 finishing by eleven

than numbers between 4700 and 4800 finishin by seventeen you will have no problem (not real anyway ) to conclude (i know i should have put eleven instead of seventeen but it would be no joke!)