Answer:
r = 54 miles per hour
Explanation:
Since we have been given the values for two of the unknowns, we can substitute them into the equation to find the third.
d = 405
t = 7.5
d = rt
405 = r (7.5)
405=7.5r
Now to find r, we need to get it by itself on one side so we have r = ...
We can do this by dividing 7.5 from both sides of the equation as this will cancel out the 7.5, leaving us with the r value, and everything you do to one side of an equation must also be done to the other side.
405/7.5=7.5r/7.5
54=r
The unknown value 'r' = 54
Since 'r' stands for 'rate', the units given must be appropriate for speed. As the distance was given in miles, and the time in hours, the unit for 'r' must be miles/hour. Therefore r=54miles/hour.
Hope this helped!