If a bat represents 15 runs and ball represents 20 runs how many balls and bat represent century

Question

$\begin{array}{11} (a)\; \text{4 bats and 1 balls} \\ (b)\; \text{3 bats and 2 balls} \\ (c)\; \text{2 bats and 5 balls} \\ (d)\; \text{4 bats and 2 balls} \end{array}$