We know that Mr. White takes three 10-minutes breaks every 6 hours.
If he travels for 36 hours, he will have 18 breaks. It's because 36 hours is 6 times 6 hours and 6 * 3 = 18.
We can also calculate that 18 10-minutes breaks is 180 minutes of breaks overall. It means that when travelling for 36 hours, Mr. White will have 3 hours of breaks.