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A friend asked me this:

A woman gave a different number of apples to each of her five children. Any three of her children together received more apples than the remaining two children. What is the least number of apples that she could have given?

I did some 'trial and error' and I think the answer is $5$. I checked this for the cases where the least number of apples were $1, 2, 3, 4$ and $5$ and in each of those cases I chose the number of apples given to the other children to be $(x+1)$, $(x+2)$, $(x+3)$, $(x+4)$ where $x$ the least number of apples given. The reason why I chose them is because if I had chosen bigger numbers, then I'll have less chance that the sum of the lowest $3$ numbers is more than the sum of the remaining $2$ numbers (the highest).

As you can see that my line of reasoning is in no way rigorous. So my question is whether there is a way to mathematically (and without trial-and-error) solve this?

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I'm not a native english speaker, but I think it should be are because what is given isn't the 'number of apples' (which is singular and motivates using is), but the apples themselves. – Git Gud Feb 16 '13 at 9:32
@GitGud Okay and thanks. – Alraxite Feb 16 '13 at 11:12
up vote 3 down vote accepted

Let $a_1<a_2<\ldots <a_5$ be the numbers of apples given in ascending order. Then there are numbers $x_i\geq0$ $\ (1\leq i\leq 4)$ such that $$\eqalign{a_2 &=a_1+1+x_1\cr a_3&=a_2+1+x_2=a_1+2+x_1+x_2\cr a_4&=a_3+1+x_3=a_1+3 +x_1+x_2+x_3\cr a_5&=a_4+1+x_4=a_1+4+x_1+x_2+x_3+x_4\cr}\quad.$$ The condition $a_1+a_2+a_3\geq a_4+a_5+1$ leads to $$a_1\geq x_2+2x_3+x_4+5\ ,$$ which shows that $a_1=5$ is the minimum possible value. We can realize it with arbitrary $x_1\geq0$ and $x_2=x_3=x_4=0$. The minimal total number of apples is then 35.

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I believe some words on why you're assuming $a_1<a_2<\ldots <a_5$ are in order. – Git Gud Feb 16 '13 at 10:33
@Git you can arrange them to be in order? – k.stm Feb 16 '13 at 10:35
@K.Stm. I meant that "some words are in order", meaning some words are required to assume $a_1<a_2<\ldots <a_5$. My issue here is with the strict inequality. – Git Gud Feb 16 '13 at 10:38
@Git Problem says "A woman gave a different number of apples to each of her five children". – k.stm Feb 16 '13 at 10:40
@K.Stm. I find that ambiguous, but I know what you mean. – Git Gud Feb 16 '13 at 10:41

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