On any given day, mail gets delivered by either Alice or Bob. If Alice delivers it, which happens with probability 1/4, she does so at a time that is uniformly distributed between 9 and 11. If Bob delivers it, which happens with probability 3/4, he does so at a time that is uniformly distributed between 10 and 12. The PDF of the time X that mail gets delivered satisfies

a) fX(9.5)=
b) fX(10.5)=

2 answers

9    10   11   `1
********
      ********
There is an overlap between the two periods. Since each individual function is uniform, the resulting f(x) is a step function such that:

f(x): (1/4)/2 = 1/8 for x∈[9,10]
f(x): (1/4)/2+(3/4)/2 = 1/2 for x∈[10,11]
f(x): (3/4)/2 = 3/8 for x∈[11,12]

∫xf(x) = 1/8+1/2+3/8=1 checks.
a) 0.125

b) 0.5