the weight of a watermelon is a random normal variable X with a mean of 200 and a variance of 50
Someone selects watermelons that weight in range of 200-k and 200+k with a probability of 60%
what is the value of k?
so, using your Z table, how many std generate 30% of the area on each side of the mean? Now multiply that by the std to get k.
You can play around with Z table stuff at
http://davidmlane.com/normal.html