When multiplying with significant figures, we use the number of digits from the least precise number. However, since the problem states that there are exactly 60 seconds in a minute, that becomes a definition, not a measurement, so we don't need to worry about the number of sig. dig. The same is true for the number of minutes in an hour and the number of hours in a day. However, 365.24 is a rounded number, so we would use the number of sig. dig in that number for our answer. Since 365.24 has 5 sig. dig., the answer would have 5 sig. dig., so 3.1557 x 10^7.