A steel manufacturer finds that the total quantity of raw materials used by his company each week is normally distributed with a mean of 1000 tonnes and a standard deviation of 200 tonnes. Calculate an estimate of the maximum quantity of raw materials the company is likely to use per week.

I thought that it is 1200 because mean is the average and so with the standard deviation it would be 1000+200=1200. But, the answer accepted '1400 or 1600 tonne', how is this calculated?