A balloon has mass of 10g in air. The air escapes from the balloon...

Given that the mass of the air $m = 10g$ and it escapes in time $t = 5s$ with velocity $v = 4 . 5cm{s}^{- 1}$.

The average force on the balloon can be given as,

$F = v\dfrac{d m}{d t}$

$⇒ F = 4 . 5 \times \dfrac{10}{5} = 9dyne$