which does give the answer 142.19.
Let me explain (1) why this formula is correct; (2) why your method is incorrect.
(1) Assuming there are 365 days in a year, the daily percent interest rate is as a decimal. So to find the total amount owing after one day, we multiply the original amount owing by . So if the original amount is $140, then after one day the amount owing is .
Each day that passes, the amount owing is times what it was the day before. So after 2 days the amount owing is:
After days the amount owing will be
So in one calendar month (= days), the amount owing is:
(2) The interest is compounded daily. So you need to calculate the amount owing at the end of each day, as I have shown you above. That means that in day 2, you'll pay a little bit more interest than you did in day 1: the interest on the interest you paid in day 1. And so on for day 3, 4 ... This extra bit each day adds up to a couple of pence by the end of the month.
Do you understand this now?