It states this:

*Allison has a bag of rice which weighs more than 17 ounces and she has a two pan balance and 4 different weights being 3 oz, 4 oz, 7 oz and 11 oz respectively. How many times at least does she has to use the balance to get 17oz of rice?*

I tried to solve this problem by summing up all the weighs:

$3\,\textrm{oz}+4\,\textrm{oz}+7\,\textrm{oz}+11\,\textrm{oz}=25\,\textrm{oz}$

Since what it is being asked is to get $17$ ounces what I tried to do is to find the possible combinations which put in each pan so that they sum up to $17$.

But grouping either $11$ in one side and $3+4+7$ in the other the resulting weight is $3$. If I choose not to use one of the weights let's say $11$ and $4+7$ it cancels both sides, while $3+7$ produces $1$ ounce in the other side therefore it cannot be used, if it is $3+4$ it produces $4$ ounces. So it seems it is impossible to get the $17$ ounces at once.

The other choice would be just using in the first attempt just $3$ and $7$, and in the second turn just the weight of $7$ ounce. Therefore the least number of times to use the balance would be $2$. But this answer is not correct.

The method I tried to use I don't think it is right, it is prone to errors and more importantly is tedious which is something I can't use at an exam where time is limited.

Can somebody help me to find a more orderly and logical method step by step to solve these kind of problems other than just guessing and starting to plug in numbers randomly as if it was trying to hit a target?.

Please don't say just the answer alone or "put the weight and the object in one pan and the rest in the other pan", I know that and It's not what I'm looking for. What I need to know is if Is there anything that can be used with more success rate such as a diagram, a table or something that I can fill up to avoid double use the weights? or maybe a suggestion such as put the numbers from the highest to the lowest and group them together. I don't know, maybe building a system of two equations?.