How many 4-digit numbers can be composed of numbers 1,2,3,4,5,6,7 if:
and the digits must not be repeated in the number
b, the number should be divisible by five, and the numbers must not be repeated
c, digits can be repeated

Correct answer:

a =  840
b =  120
c =  2401

Step-by-step explanation:

a=7 6 5 4=840
b=6 5 4 1=120

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