Remember that the probability of an event is the number of times the event can happen divided by the number of possible event:
[tex]P(A)=\frac{\#\text{ of times that A happens}}{\#\text{ of all outcomes}}[/tex]In this case, we have that the tray has 10 cakes, and 2 are strawberry. Then, the probability to randomly select a strawberry cake is:
[tex]\begin{gathered} A\colon\text{strawberry cake} \\ \Rightarrow P(A)=\frac{2}{10}=\frac{1}{5} \\ P(A)=\frac{1}{5}_{} \end{gathered}[/tex]