I have 7 black and 3 white marbles. I will randomly pick 2 of them. What is the probability that both of them are white? How to solve this?

I have 7 black and 3 white marbles. I will randomly pick 2 of them.

What is the probability that both of them are white?

How to solve this?

$\begin{array}{|rcll|} \hline && \frac{3}{10} \cdot \frac{2}{9} \\ &=& \frac{1}{15} \qquad (6.\bar{6}\ \%) \\ \hline \end{array}$

Our math teacher taught us to do 7:3 this is the ratio of black marbles to white marbles. now set it up as a fraction, 7/3. Then since you picked two marbles you will add 7 and 3 because that is your total amount of marbles. So cross multiply 7/3 and 10/2 and simplify if needed, then you have your answer to the probability.