Probability Practice Test 2

Number of red, green and white balls are 3, 4 and 5 respectively.
Probability of one red ball ${=}^3{\mathit{C}}_1$
Probability of one green ball ${=}^4{\mathit{C}}_1$
Probability of one white ball ${=}^5{\mathit{C}}_1$
Total balls = 12
So, $\frac{{}_{}^3{\mathit{C}}_1^{}{+}^4{\mathit{C}}_1{+}^5{\mathit{C}}_1}{{}_{}^{12}{\mathit{C}}_1^{}}$ $=\frac{3}{12}+\frac{4}{12}+\frac{5}{12}$