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EXAM P SAMPLE SOLUTIONS

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14. Solution: A<br />

1 11 1 1 1 ⎛1⎞ p k = pk−1 = pk−2 = ⋅ ⋅ pk−3 = ... = ⎜ ⎟ p0 k ≥ 0<br />

5 55 5 5 5 ⎝5⎠ k<br />

∞<br />

1 = ∑ pk k= 0<br />

p 0 = 4/5 .<br />

∞<br />

⎛1⎞ = ∑⎜<br />

⎟<br />

k=<br />

0⎝5⎠<br />

p0<br />

p0 =<br />

1<br />

1− 5<br />

5<br />

= p0<br />

4<br />

Therefore, P[N > 1] = 1 – P[N ≤1] = 1 – (4/5 + 4/5 ⋅ 1/5) = 1 – 24/25 = 1/25 = 0.04 .<br />

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15. Solution: C<br />

A Venn diagram for this situation looks like:<br />

We want to find<br />

( )<br />

w= 1−<br />

x+ y+z<br />

1 1<br />

We have x+ y = , x+ z = , y+ z =<br />

4 3<br />

5<br />

12<br />

Adding these three equations gives<br />

1 1 5<br />

( x+ y) + ( x+ z) + ( y+ z)<br />

= + +<br />

4 3 12<br />

2 x+ y+ z = 1<br />

( )<br />

1<br />

x+ y+ z =<br />

2<br />

1 1<br />

w= 1− ( x+ y+ z)<br />

= 1− =<br />

2 2<br />

Alternatively the three equations can be solved to give x = 1/12, y = 1/6, z =1/4<br />

⎛ 1 1 1⎞ 1<br />

again leading to w = 1−<br />

⎜ + + ⎟=<br />

⎝12 6 4 ⎠ 2<br />

k<br />

Page 7 of 55

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