Discussion
By using digits 1 to 9 how many 4 digit numbers can be formed which has 5 as a unit digit? Repetition of digits is are not allowed.
Answer: A
There are 336 such numbers.
The units digit is fixed as 5. The other three places are filled from the remaining 8 digits (1 to 9 without 5), with no repetition:
\[ \begin{aligned} \text{thousands} \times \text{hundreds} \times \text{tens} &= 8 \times 7 \times 6 \\ &= 336 \end{aligned} \]
There is no zero among the digits, so no case of a leading zero needs removing.
504 is \(9 \times 8 \times 7\), which wrongly lets 5 be used again in the first three places. 1400 matches no correct count, and 'None of these' is wrong because 336 is offered.
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