Calculating mileage of recent trip

We all take different trips contributing to our tank mileage. My trip to work is downhill and very good on mpg, then I sometimes take the highway for errands etc. Below is a simple formula that helped me calculate the mpg of a recent trip

a*b+x*c =d so x = (d-a*b )/c

where

a is mileage reading before initial trip
b is fraction of initial trip over mileage from initial and recent trip
c is fraction of recent trip over mileage from initial and recent trip and
d is total mileage

For example:
This before my work commute I had 51 miles in this tank. Going to work the odometer reads 60 miles. are needed to calculate the fractions of each trip's miles over the total miles.
So my trip to work is 15% or .15 © as a fraction of 1 of my total mileage and my cumulative miles before that trip represent 85% (b ) of the odometer reading.
The initial mpg on the MFD was 52.0 (a)
The final mpg on the MFD was 54.2 (d)

Doing the calculations from the formula above, my trip to work was 66.67mpg

It's easy going downhill