Question 291936
I was confused by your description, but I think you mean:
Jack ran 4 miles in 40 minutes, and Susan ran 3 miles in 40 minutes.
The task thus becomes one of converting to miles per hour and comparing them.
We know 60 minutes = 1 hr, so 40 minutes = 40/60 = 2/3 hr.
We also know the distance formula:  d = rt.
.
J = Jack's speed
S = Susan's speed
.
4 = J*2/3
6 = J
Jack is running at 6 miles/hr.
.
3 = S*2/3
4.5 = S
Susan is running at 4.5 miles/hr.
.
How much faster is Jack running than Susan?
1.5 miles/hr
.
Checking, how far would each run in 1 hr?
d = 6*1
Jack runs 6 miles
d = 4.5*1
Susan runs 4.5 miles
Checks.
.
Done.