\[(a)\;t=1s \quad (b)\;t=2s \quad (c)\;t=1.5s \quad (d)\;t=3s\]

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When stone is dropped from a rising balloon it has a initial velocity of $5m/s$ in upward direction.

Taking downward direction as positive and

$s=ut+\large\frac{1}{2}$$ut^2$

$30=-5 \times t+\large\frac{1}{2}$$ \times 10 \times t^2$

$5t^2-5t-30=0$

$t^2-t-6=0$

$(t-3)\; or\;(t+2)=0$

$t=3 \;or t=-2(t=-2$ negative time is not possible)

Therefore $t=3s$

Hence d is the correct answer.

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