Tali is running at 5 miles per hour.
In 12 minutes she runs 1 mile,
in 1 minute she runs 1/12 mile,
so in 60 minutes (one hour) she would run 60/12 = 5 mph
Sandra is running at 10 miles per hour, this is twice as fast as Tali (5 mph x 2 = 10 mph).
It takes Sandra 6 minutes to run a mile.
Sandra runs 10 miles in an hour, so to run one mile would take her one tenth of an hour or 6 minutes.
