Here is the question:

For a tire manufacturing company, it is known that only 7% of their tire

products last less than 35 months, whereas 11% of them last more tham 63

months. Assuming the distribution of the lifetime of the manufactured tires

is normal,

(a) what is the expected lifetime for a tire manufactured by this company,

and its standard deviation?

(b) The manufacturer company gives a warranty that if the tire does not last

more than 40 months, they will give a replacement for it. The company

sold 1000 tires during the last year. What is the probability that they

will have to replace more than 50 tires?

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So I have

a) P (X <= 35 months) = 0.07

P (X >= 63 months) = 0.11

and for b I've started with...

b) P ( X <= 40 months) = ...?

Where do I go from here, for each part?

Thank you so much in advance : )