What is the probability that a senior Physics major and then a sophomore Physics major are chosen at random? Express your answer as a fraction or a decimal number rounded to four decimal places

What is the probability that a senior Physics major and then a sophomore Physics major are chosen at random? Express your answer as a fraction or a decimal number rounded to four decimal places.

Solution:

There are a total of N = 103 students present in a Physics class.

Some of the students are Physics Major and some are not.

The instructor has to select two students at random.

The instructor first selects a senior Physics major and then a sophomore Physics major.

Compute the probability of selecting a senior Physics major student as follows:

[tex]P(\text{Senior Physics Major})=\frac{n(\text{Senior Physics Major}) }{N}[/tex]

[tex]=\frac{5}{103}\\\\=0.04854369\\\\\approx 0.0485[/tex]

Now he two students are selected without replacement.

So, after selecting a senior Physics major student there are 102 students remaining in the class.

Compute the probability of selecting a sophomore Physics major student as follows:

[tex]P(\text{Sophomore Physics Major})=\frac{n(\text{Sophomore Physics Major}) }{N}[/tex]

[tex]=\frac{20}{102}\\\\=0.1960784314\\\\\approx 0.1961[/tex]

Compute the probability that a senior Physics major and then a sophomore Physics major are chosen at random as follows:

[tex]P(\text{Senior}\cap \text{Sophomore})=P(\text{Senior})\times P(\text{Sophomore})[/tex]

[tex]=0.0485\times 0.1961\\\\=0.00951085\\\\\approx 0.0095[/tex]

Thus, the probability that a senior Physics major and then a sophomore Physics major are chosen at random is 0.0095.