:2. Solution.
SE of no. of male lawyers favoring amendment
= (.8 x .2)^(1/2) x 20 = .4 x 20 = 8
SE of % of ... = 8/400 x 100% = 8/4 = 2%
SE of no. of female lawyers favoring amendment
= (.9 x .1)^(1/2) x 10 = (.09)^(1./2) x 10
= .3 x 10 = 3
SE of % ... = (3/100) x 100% = 3%
SE of difference of percents = (2^2 + 3^2)^(1/2) = (13)^(1/2)
= 3.6%
z = (difference of % - 0)/3.6 = 10/3.6 = 2.78
The area under the normal curve between -2.78 and 2.78 is between
99.40% and 99.49%, so the area to the right of 2.78 is between
.6/2 = .3% and .5/2 = 2.5%. Now,
.3% = .3/100 = 3/1000
.25% = .25/100 = 25/10000 = 100/40000 = 1/400
The correct choice is 1/400.