Question 1183486
{{{H[0]: mu = 100}}}
{{{H[a]: mu < 100}}}

When significance level is unspecified it is normally assumed to be 5%, or 0.05.

{{{t = (X-bar - mu)/(s/sqrt(n)) = (89 - 100)/(15.81/sqrt(5)) = -1.556}}}

From stattrek.com, p-value = {{{P(t < -1.556) = 0.0973}}} for t-distribution with 4 d.f.

Since {{{alpha = 0.05 < 0.0973}}}, do not reject the null hypothesis and there is no reason to believe that average IQ is lower than 100.