Question

how many three digit numbers may be formes from the set {0, 2, 3, ... 9} if zero cannot be the first digit repitition is not allowed and A.) it is divisible by 10 B.) it is divisible by 25?

Answer 1

pls add 1 there...i just forgot

Answer 2

if i understand your problem correctly...
let's try to proceed w direct deduction -
if 0 cannot be in hundreds place then we start w numbers 100 thru 999.
by B) we narrow down the list to (9 x 4) = 36 possible numbers:
100, 125, 150, 175, 200, 225, 250, 275, 300, 325, 350, 375, ... i.e. 4 possible numbers per each hundred mark all the way to 975
now by A) we cannot repeat digits so we have to start crossing off those numbers...
we cross off 100, 200, 225, 300, 400, 500, 525, 550, 575, 600, 700, 775, 800, 900
so 36 - those 14 = 22 numbers