You can solve this by setting up 2 equations
if alice is traveling 3 times faster than brenda, we can make an equation
3x=y x is brenda, y is alice
then we can set up an equation based on d=rt
720= (x + y) x 7.5
we can plug in the 1st equation to have the same 2 variables, so for x we can plug in 1/3y or for y we can plug in 3x
720 = (x + 3x) x 7.5
720= 4x x 7.5 divide by 7.5
96 = 4x divide by 4
x= 24
brenda is traveling at 24 miles per hour. We can find alice's speed by plugging x back into the 1st equation
3x = y
3(24)= 72
Alice is going 72 miles per hour
Thanks for making the question, I had to remember how to do this