5 and 7 is not 57

Probability Level 3

Find the number of all 7-digit numbers formed from 5 and 7, which are divisible by both 5 and 7.

7 3 9 11

Vaibhav Prasad by Vaibhav Prasad , 21, India

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2 solutions

Ramiel To-ong
May 29, 2015

The actual numbers are 5557755, 5577775, 5755575, 5777555, 7555555, 7575575, 7755755, 7757575, 7775775.

0 Helpful 0 Interesting 0 Brilliant 0 Confused
Brock Brown
Feb 3, 2015

Quick Python solution: 

1
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5
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7
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9
 
from itertools import product
def goal(number):
    n = int(''.join(number))
    return n % 5 == 0 and n % 7 == 0
count = 0
for combo in product('57',repeat=7):
    if goal(combo):
        count += 1
print "Answer:", count

Running time: 0.00495 seconds

0 Helpful 0 Interesting 0 Brilliant 0 Confused

Nice... But how about you try it with pen and paper ??? :)

Vaibhav Prasad - 6 years, 4 months ago

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