In a warehouse, the inventory is stored in packages weighing no more than 1 ton each. We have a 1-ton and a 4-ton truck. What is the maximum load that we can definitely deliver in one trip?
It is easily visible that we cannot undertake the transportation of a load greater than 4 tons. Let $D$ be any positive integer, and suppose someone undertakes to transport a load of $4+D$ tons. If now in the warehouse there are packages weighing $\frac{1}{2}+d$ tons, where $0<8d<D$, then from these, at most seven can be loaded onto the 4-ton truck, and only one onto the 1-ton truck, thus in total, at most eight packages can be transported, whose total weight, $4+8d$, is less than the promised amount.

If we only undertake to transport 4 tons, then we can certainly fulfill this promise. Start by loading the 4-ton truck, and place the first package that no longer fits onto the other, the 1-ton truck. This will certainly fit, and the two trucks together will carry more than 4 tons of cargo. We have already over-fulfilled our promise, however, as we have seen, we cannot guarantee this surplus in advance.

Remark. Similarly, it can be proven that under the given conditions, with a 1-ton and a $t$-ton truck, we cannot undertake to transport more than $t$ tons of cargo, but in every case, we can load more than $t$ tons onto the two trucks.