Well, 9:00 AM to 4:00 PM would be 7 hours. 7 hours into minutes would be 420. 420 divided by 20 is 21. This means it multiplies 21 times from 9:00 to 4:00. Since it multiplies by 2, than you need to skip count with the pattern multiply by 2 and stop once you have 21 numbers.
1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576. That would be 21 numbers.
The answer would be 1048576.
I think this is correct. :/