H0 : mu1 - mu2 = 0
HA: mu1 -mu2 not equals to 0
test statistics:
z = (x1 -x2)/sqrt(s1^2/n1+s2^2/n2)
= ( 521.54 - 496. 92)/sqrt(101.52^2/534 + 96.47^2/511)
= 4.020
p value = 0.00005
Reject H0 at 0.05 alpha
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