Question

what is a number of 3 digits and multiple of 5 but less than 600 and can be formed from 1,3,5,7 and 9?

Answer 1

any multipule of 5 ends with either a 0 or a 5

so we see that ther are no 0's in the list so it must end with 5

so the other 2 digits can be any number
wait, less than 600
so the first digit can be 1 or 3


some possible numbers are

135
175
195
315
375
395

those are the possible numbers