A number theory problem by Victor Herlys

When you remove the eggs that are in a basket 2,3,4,5,6 each time , remains 1,2,3,4,5 eggs respectively. When they are removed 7 by 7, don't remain any eggs in the basket. What is the small number of eggs that a basket in this conditions may contain?


The answer is 119.

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1 solution

With N N being the number of eggs in the basket, we have that

N ( k 1 ) ( m o d k ) N \equiv (k - 1)\pmod{k} for each of k = 2 , 3 , 4 , 5 , 6 k = 2, 3, 4, 5, 6 , and so

( N + 1 ) 0 ( m o d k ) (N + 1) \equiv 0\pmod{k} for each of these values of k k .

Thus N + 1 N + 1 must be divisible by each of 2 , 3 , 4 , 5 2, 3, 4, 5 and 6 6 . The least positive value that meets these conditions is LCM ( 2 , 3 , 4 , 5 , 6 ) = 60 (2,3,4,5,6) = 60 , and thus the least possible value for N N is 59 59 , and in general the possible values for N N are of the form 59 + 60 m 59 + 60m for any non-negative integer m m .

Now we must deal with the condition that N N is divisible by 7 7 . We thus need to find the least value of m m such that 59 + 60 m 59 + 60m is divisible by 7 7 . This is the case when

59 + 60 m 0 ( m o d 7 ) 60 m 59 ( m o d 7 ) ( 56 m + 4 m ) 4 ( m o d 7 ) 4 m 4 ( m o d 7 ) 59 + 60m \equiv 0\pmod{7} \Longrightarrow 60m \equiv -59\pmod{7} \Longrightarrow (56m + 4m) \equiv 4\pmod{7} \Longrightarrow 4m \equiv 4\pmod{7} ,

the least positive solution for which is m = 1 m = 1 .

Thus N = 59 + 60 1 = 119 N = 59 + 60*1 = \boxed{119} .

Nice solution a bit different than mine, I used the Chinese remainder theorem, but the result is the same (:

Victor Herlys - 6 years, 8 months ago

since you solved it... i believe you understand the question, can you explain the question...

Kap Son - 5 years, 11 months ago

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