Let us see the pattern
2, 4, 8, 16, 32,
Actually it is the powers of 2
2^1 = 2
2^2 = 4
2^3 = 8
etc
It is 6 hours and as 1 hour = 60 mins so 6 hours = 60 * 6 = 360 minutes
Now we will divide 360 by 20 which comes to 18
Now 2^18 is 262144 {I have copied this from calculator not from internet}
So there would be 262144 bateria after that.