Answer:
36 mph
Explanation:
The formula you want to use is:
distance = speed * time
Let's assume
t = time they both drove
j = speed of Jenny
m = speed of Mary
Then you can translate the stated facts into these equations:
240 = j * t
180 = m * t
j = m + 12
Rewrite the first two as:
t = 240/j
t = 180/m
and eliminate t (since they both drove the same time):
240/j = 180/m
Rewrite as (cross multiply):
240m = 180j
Now fill in the third and simplify:
240m = 180(m+12)
240m - 180m = 2160
60 m = 2160
m = 36.
Bonus:
Jenny drove 36+12 = 48 mph
It took them 240/48 = 180/36 = 5 hours