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John, Mike, Emily and Nick want to buy some apples from Walmart where the apples are sold in pack of six. How many packs should they buy to get the same amount of apples for each one of them? 
Overseas Competition->Knowledge Point->Number Theory Modules->Factors and Multiples
For each one of them to get the same amount of apples, the total amount should be a multiple of $4$, because there are $4$ of them.  The apples are sold in pack of six. $3$ packs contain $18$ apples; $5$ packs contain $30$; $6$ packs contain $36$; $7$ packs contain $42$.  Among the number of $18, 30, 36, 42$, only $36$ can be divided by $4$. $36$ is a multiple of $4$. Therefore, they should buy $6$ packs. 