To find how far it could travel in 15 hours, divide 15 by 5, giving you three. You then have to multiply 460 by 3, giving you 1380.
In order to find the rate in an hour, you have to divide both 'sides' (the 460 miles in 5 hours) by 5, and this will give you the rate per hour. 460/5= 32.
Therefore, in 15 hours, the plane is able to travel 1380.
The speed of the plane per hour is 92 miles/hour.
Hope this helps :)