Since Dharmesh walked 1/2 mile in 20 minutes, and 20 minutes is 1/3 of an hour, multiply how much he walked in 20 minutes by 3 to get the speed in 1 hour.
Multiply by 3 because 1/3 of an hour is 20 minutes. If you switch it around, it becomes 20*3
1/2 * 3 = 1.5 or 1 1/2.
Hope this helped☺☺