To solve this we use the z statistic. The formula for z score is:
z = (x – u) / s
where x is the sample value = less than 16, u is the sample mean = 16.1, s is standard dev = 0.1
z = (16 – 16.1) / 0.1
z = - 1
From the standard distribution tables at z = - 1,
P (z = -1) = 0.1587
So about 15.87% is underweight.