A scaled frequency histogram of data values shows that they

A scaled frequency histogram of data values shows that they have a mean of zero, and a standard deviation of 2. Roughly, what would be the best estimate of the sum of the values for all the bins from -2 to 2?  

a. 0            b. 4                 c. 0.7<--ans              d. -2                 e. 1

how did they come to this awnser?

Solution

P(-2<X<2) = P(-2/2 <(X-mean)/s <2/2)

=P(-1<Z<1) =0.6827 (from standard normal table)

Answer: c. 0.7<--ans