So basically everything in my working is correct except that I shortcutted and didn't convert days-1 to seconds-1, causing me to mess up at the final step because it should be 0.07 ÷ (24 x 60 x 60) to convert days-1 to seconds -1.

At first I got 0 marks but then I asked my teacher and she gave me one mark. Apparently I'm "always supposed to use seconds" and also the mark scheme said I had to find initial N first, and then only use the logarithmic function for N to find A, so my working to find A straight away didn't score any marks. Also the mistake meant that I was dividing by a much larger value, so my answer was way off the mark, which my teacher took to mean that my whole working was off the mark.

Now clearly I deserve 2 or 3 marks here but what is the likelihood that an external marker grading my paper also has poor mathematical understanding and doesn't understand that I can use days-1 since t is in days?? I don't mean to disrespect any teachers but I know many teachers just follow formulas and rote learning. Should I just take more time, convert decay constant to seconds, convert half life to seconds, convert everything to seconds in every step in case my external markers don't understand basic shortcuts?