Question 924516
mean 278 
 {{{z =blue (x - mu)/blue(sigma/sqrt(n))}}}
z = invNorm(.86) = (280-278)/(sd/√36)
1.0803 = 2/(sd/6)
(sd/6)(1.0803)  = 3
sd = 18/1.0803 = 16.6620 rounded  (use the number of decimal points as directed)