$\textbf{Make Four Liters}$
If you have an infinite supply of water, a $5$-liter bucket, and a $3$-liter bucket, how would you measure exactly $4$ liters of water? The buckets do not have any intermediate scales.
$\textbf{Solution}$
First fill the the $5$-liter bucket and use the water to fill the $3$-liter bucket. This will leave $2$ liters of water in the bigger bucket. Then empty the smaller bucket and transfer the $2$ liters of water from the bigger one to the smaller one. The final step is to fill up the $5$-liter bucket and then use the water to fill up the $3$-liter bucket. Because there are already $2$ liters of water in the $3$-liter bucket, therefore only one liter is needed to fill it up. This will leave $4$ liters of water in the bigger bucket.