Question

A company tested 735 of the lightbulbs they produced and found them to have a mean life of 1,200 hours and a standard deviation of 50 hours. How many of these lightbulbs had a life between 1,170 hours and 1,230 hours?

Answer 1

$P(1170\le X \le1230)=\\ P(\frac{1170-1200}{50}\le Z \le\frac{1230-1200}{50})=\\ F(\frac{1230-1200}{50})-F(\frac{1170-1200}{50})=\\ F(0.6)-F(-0.6)=\\ 0.7257 -0.2743 =\\ 0.4514$