Question

7. Safa is a resident of UAE where mobile numbers consist of 7 digits. She tells her friend that

her mother’s mobile number is the smallest number and father’s is the greatest number that
can be formed by using the digits 5,7,9,8. Help her friend to find her parents numbers. Each
digit must be used at least once.
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Answer 1

Explanation:

7 digits in a number.

each digit of 5, 7, 9, 8 must be in it at least once.

but (as I understand it) no other digits can be used.

for the smallest number we start with the smallest digit for the highest ranked places, and then we fill up with the other 3 digits sorted by size low to high :

5555789

similarly, for the largest number we start with the largest digit for the highest ranked places, and then we fill up with the other 3 digits sorted by size high to low :

9999875