The Hourglass in a Box

Riddle 
A box contains 10 different hourglasses, each with a different sand timer value. You randomly pick one hourglass, turn it over, and let it run out. What is the probability that you picked the hourglass that takes the longest time to empty?

    1. Start both hourglasses at the same time. 2. When the 4-minute hourglass runs out, flip it immediately. 3. When the 7-minute hourglass runs out, flip it immediately. 4. When the 4-minute hourglass runs out again (at 8 minutes), there is 1 minute of sand left in the 7-minute hourglass. 5. Let that 1 minute pass. You’ve now measured exactly 9 minutes.