# How to model shipment size constraint?

I am working on an LP problem where I have to model a constraint as:

"The total number of units of product A and B should be shipped in multiples of $$1200$$"

e.g. $$700\text{(product A)} + 500\text{(product B)} = 1200\times1 = 1200$$ or

$$1800\text{(product A)} + 600\text{(product B)} = 1200\times2 = 2400$$ or

$$2000\text{(product A)} + 1600\text{(product B)} = 1200\times3 = 3600$$

like that maximum shipment should be $$6000$$ units $$(1200\times5)$$, or I can say in other words as I can only ship $$5$$ batches of $$1200$$ units each.

Can somebody please help me to write such a constraint in algebraic form: $$Xa + Xb =$$?

If $$x_p$$ denotes the quantity of product $$p\in P$$ that is shipped : \begin{align} \sum_{p\in P}x_p &= 1200 k \\ k &\in \mathbb{N} \\ x_p &\in \mathbb{R}^+ \end{align}
• @TheSimpliFire : just out of curiosity, why is better to write equations with \begin{align}, rather than with '' symbols ? Jan 20 '20 at 19:28 • Well because it aligns equations together :) (note that and \\ newline do not achieve this as there is no command to align $=$ and $\in$ in this case) Jan 21 '20 at 7:26