# What weights should I buy for my gym? (a case of integer partitioning)

I am trying to solve for possible combinations of weights that would be appropriate for use in my home gym. I have been told that this is a case of integer partitioning, but I am not sure how to solve it and would appreciate some guidance (or even better a solution!).

Given that I am quarantined at home, I want to buy weight plates for use in a home gym. In my gym, I have a 45lb bar. I attach weight plates to the bar in pairs, so I always purchase and use plates in pairs. So, if I buy 1 pair of 2.5lb plates, I would be able to use the bar at weights of 45lb or 50lb (since plates must always be used in pairs).

I want to determine how many pairs of 2.5lb, 5lb, 10lb, 25lb, and 45lb plates I need to ensure that I can use my bar at weights of 45 to 405lb in 5lb increments (so options would include 45, 50, 55, 60,..., 395,400, 405). I want to minimize the number of smaller plates that I buy (so mostly get 45s and then get some minimum number of pairs of each of the smaller plates to ensure all possible combinations are attainable).

How might I approach this problem?

• This is amost the same as a light bulb problem I have seen, where the aim is to be able to produce any desired brightness from a set of lamps. I have a set of weights for you, if you care to come and pick them up. (They are too heavy for me to lift.) I only need one or two of them for cold pressing pitch laps. Consider a T-Rex strap in a door. No weights other than your own body, and a very versatile workout device. But set it up so that you fall in a soft place if you lose your grip, if something breaks, or if something comes loose from the door. – richard1941 Mar 25 at 0:45
• I'd like to mention that this wouldn't be a problem if you bought kilogram plates instead. There is a unique way to load every multiple of 2.5 kg above 20 kg if you buy one pair each of 1.25 kg, 2.5 kg, 5 kg, and 10 kg plates, and enough 20 kg plates to reach your maximum weight. Consequently, you would save the cost of one pair of plates if you switched to metric, assuming kilogram and pound plates cost approximately the same. – Samuel Mar 27 at 4:09

To make it up to $$405$$ you could buy: $$\begin{array}{r|ccccc} \text{weight}& 2.5&5&10&25&45\\ \hline \text{pairs}& 1&1&2&1&3 \end{array}$$ The idea is that in any combination of weights you can often replace multiple small weights by a single large weight. For example $$2$$ pairs of $$2.5$$'s can be replaced by a $$5$$, so you will never need $$2$$ pairs of $$2.5$$'s. Similarly you will never need $$2$$ pairs of $$5$$'s, or $$3$$ pairs of $$10$$'s, or $$2$$ pairs of $$25$$'s.

You will need $$2$$ pairs of $$10$$'s for some combinations, such as $$90=45+2\times(2.5+2\times10).$$ Finally, just buy enough pairs of $$45$$'s to get up to the desired weight of $$405$$. These weights get you all the way up to $$420$$ even.

Finally, instead of $$2$$ pairs of $$10$$'s you could also buy $$2$$ pairs of $$5$$'s and $$1$$ pair of $$10$$'s to reach all combinations. This may save some money, while still getting you up to $$410$$ total.

• @WeightlifterPerson I have added some clarification for the reasoning; upper bounds for each of the weights are easy to find. Then a (few) routine checks to see how many you actually need. – Servaes Mar 23 at 16:28
• Thanks a lot for your help! A lot of stuff is getting sold out quickly, so this speeds up the buying process. – WeightlifterPerson Mar 23 at 16:30
• @WeightlifterPerson No problem. Instead of $2$ pairs of $10$'s you could also buy $2$ pairs of $5$'s and $1$ pair of $10$'s to reach all combinations. This may save some money, while still getting you up to $410$ total (instead of $420$). – Servaes Mar 23 at 16:33
• I like the way you think. A solution and an opportunity to save money - doesn't get better than that! – WeightlifterPerson Mar 23 at 16:39
• If you provide a cost for each pair, you can instead find a minimum-cost solution, presumably subject to some constraint on the number of pairs that can fit on a bar at once. – Rob Pratt Mar 23 at 17:52

Let's say you want to obtain the values $$v_1,...,v_n$$ as a sum of nonnegative integer multiples of $$w_1,...,w_k$$.
I.e. you're looking for a vector $$(a_1,..,a_k)$$ so that $$\forall i\in [n]: \exists b_1\in[ a_1],..., b_k\in [a_k]: \sum_{j=1}^n b_j\cdot w_j = v_i$$ This formulation already shows a way to brute-force yourself to the solution: "function" returns the minimal vector (towards the $$||\cdot ||_1$$ norm) that fulfills the constraints.

The set $$\{\left(a_1,..,a_k\right)\in Z^k\mid a_1+\ldots+a_n=m\}$$ is the set of weak compositions of $$m$$ into $$n$$ parts.