Well, I would use this general result on 'combining errors' in things that are normally distributed:
If you have a variable normally distributed about a mean value with variance , and a function , the function will be approximately normally distributed about with a variance
In this case, the variable is the length with
The function is the period given by:
You need to find the standard deviation where .
Use in that expression.
...and then, using the properties of the normal distribution, you can find the probability of being wrong by a certain amount (enough to make the clock slow down by a minute a day). There's quite a bit of calculating left for you to do, but hopefully that should help you on your way.
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You may have a different way to do it... it really depends on what you've been taught.