Suppose that a shop has an equal number of LED bulbs of two different types. ==> Therefore
Probability of Taking Type 1 Bulb => 0.5
Probability of Taking Type 2 Bulb => 0.5
The probability of an LED bulb lasting more than 100 hours given that it is of Type 1 is 0.7, and given that it is of Type 2 is 0.4.
Prob(100+ | Type1) => 0.7
Prob(100+| Type2) => 0.4
Prob(100+) => Prob(100+ | Type1) * Prob(Type1) + Prob(100+ | Type2) * Prob(Type2)
= 0.7 * .5 + .4 * .5
= 0.55