24
33
36
27

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By the question we have,

20% heard 60 minutes.

10% heard 0 minutes.

Half the remainder , $= \frac{70\%}{2} = 35\%$ heard 20 minutes

And the other 35% heard 40 minutes.

Therefore average number of minutes heard $= 20\% \times 60 + 10\% \times 0 + 35\% \times 20 + 35\% \times 40$

$= 0.2 \times 60 + 0 + .35 \times 20 + .35 \times 40$

$= 12 + 7 + 14$

$= \boxed{33}$