Using the digits 1 through 9, find the number of different 4-digit numbers such that: (a) digits can be used more than once.
(b) digits cannot be repeated. 2 .
(c) digits cannot be repeated and must be written in increasing order.

b

step-by-step explanation:

54 km / hours

step-by-step explanation:

we need to convert seconds to hours

60 seconds = 1 minutes

60 minutes = 1hour

15 m         60 second   60 minutes

  * *         = 54000 meters/ hour

second     1 minute             1 hour

now we need to convert meters to km

1000 m = 1 km

54000 m       1 km

*   = 54 km / hours

hour               1000m

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