Question 15
HardOriginal questionHow many three-digit numbers that are divisible by 5 have all three digits different?
Show the answer and explanation
Correct answer: C
Explanation
Split by the units digit, since the rule for 0 changes with it.
Units digit 0: hundreds digit has 9 choices, tens 8, so .
Units digit 5: hundreds digit cannot be 0 or 5, so 8 choices; tens then has 8 choices (0 is allowed), so .
Total .
Worth noting
128 treats both cases like the units-5 case; 180 counts every three-digit multiple of 5, with repeats allowed. The two cases must be counted separately because 0 is barred from the front only in one of them.
- Concept
- Case-splitting when a restriction interacts with the zero digit
- Typical timeTypical
- 120 s
- Your timeYours
- —
Ten questions from Counting and Basic Probability, at your own pace.