20 minutes = 1/3 of an hour
1/3 = 0.333
1/3 x 45mph = 15, so she drove 15 miles at 45mph
18-15 = 3
3 miles/ 20 mph = 0.15 hours
0.333+0.15 = 0.483 hours total
0.483 x 60 = 28.98 so approximately 29 minutes total driving time
7:15 + 29 minutes is 7:44 am she arrived at work
A. assuming the sales tax is already added to the 72.60 all you have to do is add 20%
I just agree with the other comment
To prove:

Solution:

Multiply first term by
and second term by
.

Using the identity: 

Denominators are same, you can subtract the fractions.

Using the identity: 

Using the identity: 

------------ (1)

Using the identity: 


------------ (2)
Equation (1) = Equation (2)
LHS = RHS

Hence proved.