At what time between 8 and 9 o'clock will the hands of a watch be in straight line but not together ?

At 8 o'clock, the hands of the watch are 20 min. spaces apart. 

To be in straight line but not together they will be 30 min. space apart.

$\therefore$Minute hand will have to gain 10 min. spaces 

55 min. spaces are gained in 60 min.

10 min. spaces will be gained in

$\left(\frac{60}{55}\times 50\right)\min$ or $10\frac{10}{11}\min$

$\therefore$Required time $=10\frac{10}{11}\min .$ past 8