Step 2: When the 7-minute hourglass is about to leak, then make a 7-minute hourglass B;

Step 3: When the hourglass leaks to 10 minute, start timing immediately;

Step 4: When the second 7-minute hourglass leaks B, the time is 4 minutes;

Step 5: Followed by two 7-minute hourglasses, after the leakage, the time is 18 minutes.