To find out how much water Yosef drank, we can calculate it using the information provided in the question.
Yosef drank Two-fifths of a glass of water every hour for 10 hours. We can write this mathematically as:
\[ \text{Total Water} = \left(\frac{2}{5} \text{ glass/hour}\right) \times (10 \text{ hours}) \]
Now, perform the multiplication:
\[ \text{Total Water} = \frac{2}{5} \times 10 = \frac{2 \times 10}{5} = \frac{20}{5} = 4 \text{ glasses} \]
So, the total amount of water Yosef drank is 4 glasses.