The difference in the number of ratio units is 5-3 = 2. This corresponds to an original difference of 2+2 = 4, so each ratio unit represents 2 pieces of fruit.
Or, if you're familiar with the empirical rule and the basic properties of the normal distribution, you can use the fact that approximately of the distribution lies within two standard deviations of the mean, or
which means about of the distribution lies outside this interval, or