$11 、 \mathrm{~A}, \mathrm{~B}, \mathrm{C}$ are 3 prime numbers less than 20, $A+B+C=30$, find these three prime numbers
Because $A+B+C=30$ is an even number, $\mathrm{A}, \mathrm{B}, \mathrm{C}$ must be two odd and one even or all even. Since $\mathrm{A}, \mathrm{B}, \mathrm{C}$ are prime numbers less than 30, and 2 is the only even prime number, they must be two odd and one even. Therefore, one of them must be 2, and the other two are 11, 17.