Rayna is building a tapered staircase using wooden boards. The bottom stair is 3 1/2 ft wide, and the top stair is 2 1/6 ft wide. Each stair is 4 in. Shorter than the one below. What is the total length of wooden boards rayna needs for the stair tops, in feet? Enter your answer as an improper fraction.

Do the same process for each of the 3 

you have miles per how many minutes and you need miles per hour
you know 60 minutes make 1 hour  convert the minutes to hours 
this will gives you miles per equivalent hour
you need miles per 1 hour 

Here is number 1  
1) 5 miles in 20 minutes 
    5 miles in (20/60) hr  = 5 miles in 1/3 hr 
to get 1 hr multiply by 3 
equation so keep the balance 
5 * 3 = 15 
rate = 15 miles per hour.

same process for the others