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# Probability

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A jar contains 5 pennies, 6 nickels and 5 dimes. A child selects 2 coins at random without replacement from the jar. Let X represent the amount in cents of the selected coins.

Find the probability X = 10.

Find the expected value of X

Apr 12, 2021

### Best Answer

#1
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First question:

The child will only get 10 cents if he selects 2 nickels. The probability of that happening is $$\frac{{6 \choose 2}}{16 \choose 2}=\frac{15}{120} = \boxed{\frac{1}{8}}$$

Second question:

This is basically using the same logic as the previous question.

X could be equal to 2, 6, 10, 11, 15, or 20, and their probabilities are $$\frac{{5 \choose 2}}{{16 \choose 2}}, \frac{5\cdot6}{{16 \choose 2}}, \frac{{6 \choose 2}}{{16 \choose 2}}, \frac{5\cdot5}{{16 \choose 2}} , \frac{5\cdot6}{{16 \choose 2}},$$ and $$\frac{{5 \choose 2}}{{16 \choose 2}}$$, respectively. Multiply the value of X to the corresponding probability to get:

$$2\cdot\frac{{5 \choose 2}}{{16 \choose 2}}+ 6\cdot\frac{5\cdot6}{{16 \choose 2}}+ 10\cdot\frac{{6 \choose 2}}{{16 \choose 2}}+ 11\cdot\frac{5\cdot5}{{16 \choose 2}} + 15\cdot\frac{5\cdot6}{{16 \choose 2}} + 20\cdot\frac{{5 \choose 2}}{{16 \choose 2}}\\= \frac{1}{12}\cdot(2+20)+\frac{1}{8}\cdot10+\frac{1}{4}\cdot(6+15)+\frac{5}{24}\cdot11\\=\boxed{\frac{85}{8}}$$

Apr 12, 2021

### 2+0 Answers

#1
+420
+1
Best Answer

First question:

The child will only get 10 cents if he selects 2 nickels. The probability of that happening is $$\frac{{6 \choose 2}}{16 \choose 2}=\frac{15}{120} = \boxed{\frac{1}{8}}$$

Second question:

This is basically using the same logic as the previous question.

X could be equal to 2, 6, 10, 11, 15, or 20, and their probabilities are $$\frac{{5 \choose 2}}{{16 \choose 2}}, \frac{5\cdot6}{{16 \choose 2}}, \frac{{6 \choose 2}}{{16 \choose 2}}, \frac{5\cdot5}{{16 \choose 2}} , \frac{5\cdot6}{{16 \choose 2}},$$ and $$\frac{{5 \choose 2}}{{16 \choose 2}}$$, respectively. Multiply the value of X to the corresponding probability to get:

$$2\cdot\frac{{5 \choose 2}}{{16 \choose 2}}+ 6\cdot\frac{5\cdot6}{{16 \choose 2}}+ 10\cdot\frac{{6 \choose 2}}{{16 \choose 2}}+ 11\cdot\frac{5\cdot5}{{16 \choose 2}} + 15\cdot\frac{5\cdot6}{{16 \choose 2}} + 20\cdot\frac{{5 \choose 2}}{{16 \choose 2}}\\= \frac{1}{12}\cdot(2+20)+\frac{1}{8}\cdot10+\frac{1}{4}\cdot(6+15)+\frac{5}{24}\cdot11\\=\boxed{\frac{85}{8}}$$

textot Apr 12, 2021
#2
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Oh I also forgot to add in the "Find the probability X = 11." Sorry, can you help me with that ?

igggyuh  Apr 12, 2021
edited by igggyuh  Apr 12, 2021