If you could explain these to me, that would be greatly appreciated!

87. 

Since there are three digits and each digit has 10 options,

there are a total of:

10^3 ways of identifying students. 

The first number can be a 0, since it is not supposed to be a number, just an identification. 

10^3 = 1000

That is you final answer. 

88.

There are a total of:

\(\binom{15}{4}\)

ways to choose the debaters. 

This is equal to 1365.

Since we need those four people, with only one way to choose them.

The chance this will happen is 1/1365. 

89. The probability of choosing a dime first is \(\frac{2}{10}=\frac{1}{5}\) .

Next, picking a quarter without replacement is just \(\frac{3}{10}\).

Multiplying, we get \(\frac{1}{5}*\frac{3}{10}=\frac{3}{50}\)

90.

The total number of marbles: \(12+8+4+16=40\)

Blue Marble: \(\frac{4}{40}=\frac{1}{10}\)

Red marble: \(\frac{12}{40}=\frac{3}{10}\)

So, the answer is just \(\frac{1}{10}*\frac{3}{10}=\boxed{\frac{3}{100}}\)