Energy of eclipsed and staggered conformations of ethane and propane
That diagram is horribly drawn.  The way it is drawn, the 13.6 kJ/mol number is meaningless - it measures from the maximum of one curve to the minimum of the other curve.  

I think that you are supposed to assume that for one of the molecules, the one for the lower curve, the torsional strain is 12.6 kJ/mol and for the other molecule the torsional strain is 13.6 kJ/mol.  

Do you have some reason to think that the barrier to free rotation for propane would be higher or lower than for ethane?  Perhaps your book gives the value for the torsional strain of ethane/propane? (through a quick google search I was able to find the torsional strain for both of these)