A probability of 1/10 means that you expect one in ten spins to land on 2.
Performing 300 spins is the same as performing 10 spins, 30 times.
You expect to have a 2 every 10 spins, so if you repeat this process 30 times, you expect to have 2 thirty times.
In general, if you know that you have probability of success p, and you repeat the experiment n times, you expect a number of success equal to np.
In this case, p=1/10, n=300, so the expected number of successes is
[tex]300\cdot\dfrac{1}{10}=30[/tex]
