Stuck on Normal Distribution question (OCR A)

I was able to do the first bits of this question but was stuck on the last part, here is the main info for the question:

The masses of 1000 pebbles on a beach are normally distributed by the random variable X~N(200,60²)

The smallest and largest masses of the pebbles in the sample were 112g and 288g respectively. If you put this into a calculator P(X>288)=0.0712(3sf) so the model is not appropriate as it does not equal zero.

The question is asking me to suggest a different value for the standard deviation in light of these results

Think it's something to do with Z-Scores, but I'm not sure. Please help!

The 1000 pebbles is a bit of a hint. What percentage of data lies withing
+/- one standard deviation of the mean
+/- two standard deviatinos of the mean
+/- three standard deviations of the mean

You could similarly use either the min or max values with a z = 0.001 or 0.999 as that would represnt the first or last 1/000th data point.