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Popular Word Problems >

pair of dice is rolled. What is the probability that the sum is nine or that the first number is a two?

Solution

parity

o f d i ce i s ro ll e d . Wha t i s t h e p ro babi l i t y t ha t t h e s u m i s nin e or t ha t t h e f i rs t n u mb er i s a tw o ?

Solution

\frac{5}{18}

+1

Decimal

0.27777 \dots

Solution steps

Find the probability of the sum is 9:

\frac{1}{9}

Find the probability of that the first number is a 2:

\frac{1}{6}

Since the events are disjoint, Use the formula

P (X_{1} \lor X_{2} \lor \dots \lor X_{k}) = P (X_{1}) + P (X_{2}) + \dots + P (X_{k})

= \frac{1}{9} + \frac{1}{6}

\frac{1}{9} + \frac{1}{6} = \frac{5}{18}

= \frac{5}{18}

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