Machine math time hilfe

Question

The assembling of a machine consists of 2 steps S1 and S1. The duration
D1 of S1 follows a uniform distribution over [5 min,8 min] and the
duration D2 of step S2 follows a uniform distribution over [10 min,15
min]. The random variable D1 and D2 are assumed to be independent.
(a) Determine the expected values and variance of D1 and D2.
(b) 100 machine are assembled. Determine an approximate distribu-
tion of of total assembling time D.
(c) The working time of a worker is 8 hours per day. Determine the
probability that one worker assembles 100 machine in four days.

Answer 1

a)

Erwartungswerte von D1 und D2 addieren, Varianz so wie in https://de.wikipedia.org/wiki/Stetige_Gleichverteilung#Varianz

b)

Werte aus a) mal 100.

c)

Wird nach exakt 100 oder mindestens 100 gefragt?