If you used slowmotion to record the balloon, we need to know how long a slowmotion video second lasted in reality:
Look at your 10 s video of the clock, or make it with the same camera.
* See how long 10 seconds on the clock lasts in slowmotion time
* Divide the amount by 10 --> you have the Slowmotion factor
(f.i., it lasts 50 seconds, then your slowmotion factor is 50/10=5)
* Now divide the time your balloon in slow motion takes by the slowmotionfactor
(
f.i., slowmotion factor =5. Balloon takes 13 seconds in slowmotion, then 13/5=2,6 s in reality)