Let’s call the 7 minute hourglass A and the 4 minute hourglass B.

Start the 2 hourglasses simultaneously.

When B has been emptied, flip it. 4 minutes have passed since the start.

When A has been emptied, flip it. 7 minutes have passed.

When B has been emptied again, we’re at the 8 minute mark, and 1 minute worth of sand should have flowed down in A. Flip A.

When A is emptied, we are at 9 minutes.

7-4=3 ✓

4-3=1

7-1=6 ✓

6+3=9 ✓

【9＝4+4+1】。4♻️4+4 ， ➕＝7♻️1，♻️1。lightning:cndx@nostrurl.com 🐰ᥬ[🐕]᭄🌿

7（-4）3，4（-4）0

Start

3（-3）0，turn 4（-3）1， +3

Turn 7（-3）4，turn 3（-3）0， +3

Turn 3（-3）0，0（no operation）0， +3

So

3+3+3=9