I borrowed a MTB tandem to take my VI stoker for his first off road ride the other day, half of the ride was on a trail the other was on the road, best leg workout I've had for awhile, in fact it wrecked me for a few days.
My guess the sum would be if you rode a tandem, solo roadie, MTB and track bike on the same flat road for 20 miles each...
20m solo= 25 tandem=30 MTB= 22 track?