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Counting and Basic Probability · Question 15 of 65

Question 15

HardOriginal question

How many three-digit numbers that are divisible by 5 have all three digits different?

Options for question 15

Show the answer and explanation
CorrectNot quiteAnswer C

Correct answer: C

Explanation

  1. Split by the units digit, since the rule for 0 changes with it.

  2. Units digit 0: hundreds digit has 9 choices, tens 8, so 9×8=729 \times 8 = 72.

  3. Units digit 5: hundreds digit cannot be 0 or 5, so 8 choices; tens then has 8 choices (0 is allowed), so 8×8=648 \times 8 = 64.

  4. Total =72+64=136= 72 + 64 = 136.

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 time
120 s
Your time
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The rule this testsThe counting principle
Practise this chapter →

Ten questions from Counting and Basic Probability, at your own pace.

Practise this chapter →

Ten questions from Counting and Basic Probability, at your own pace.