Well, the only way I got that answer is by doing this:
you want to find out what mass x has an uncertainty equal to 1 grain, aka a value of anywhere from 60 and 65mg.
The range of uncertainty from 60 to 65mg is 5mg.
If you calculate the percent of uncertainty you get:
5/60 = 8.3333%
5/65 = 7.6923%
If you do 65/the larger uncertainty, you get 65 * (60/5) = 780
If you do 60/the smaller uncertainty, you get 60 * (65/5) = 780
I kind of worked backwards from the answer, but even my own explanation doesn't make much sense to me lol. Maybe this paired with your own solution from the problem will help? Or maybe someone else can help explain what I did🙂
Tbh, I don't think you'll get a problem as hard/tricky as this one though.