speed = distance / time

Average speed also = distance / time

Assume two chunks of time: t1 spent driving at 97 km/hr and t2=26.4min spend driving at 0 km/hr

Convert break to hours: 26.4/60 = 0.44 hr

Average the speeds: (s1 + s2) / (t1 + 0.44)

(97 + 0) / (t1 + 0.44) = 64

t1 = 97/64 - 0.44 = 1.075625

Or, total time = 1.515625 hr and total distance = 64 km/hr * 1.075625 hr = 68.84 km.

But, I may have done it wrong. It's late and I haven't checked for mistakes...