day 1 - $.1 - get baseline fee
day 2 - $.2 - you get charge double the previous day b/c its still less than .3
day 3 - $.3 - you can't get charged double the previous day (.4) because .3 results in a lesser amount
day 4 - $.3 - same as day 3

total = .9

The answer was .70

I chose .90 based on the calculation you mentioned in ur reply ...

tutonaranjo wrote:day 1 - $.1 - get baseline fee
day 2 - $.2 - you get charge double the previous day b/c its still less than .3
day 3 - $.3 - you can't get charged double the previous day (.4) because .3 results in a lesser amount
day 4 - $.3 - same as day 3

total = .9

The error is on day 2.

day 1: 10 cents

day 2: the TOTAL fine is doubled, which means we add an extra 10 cents... total fine is now 20 cents

day 3: TOTAL fine is doubled, so we add an extra 20 cents... total fine is now 40 cents

day 4: Doubling would add another 40 cents, so we add the maximum 30 cents... total fine is now 70 cents: choose (b)