A question about architect! ( cache , speedup , etc )

[zc0000]

30. Changing a certain cache from direct mapped to two-way set-associative caused the hit ratio measured for a certain set of benchmarks to improve from 0.91 to 0.96, with TC = 20 ns, and TM = 70 ns. What is the speedup, assuming no change in TC from the set-associative multiplexer? Speedup is the ratio of the time to complete a process without the improvement to the same process with the improvement.
(A) 1.00 (B) 1.11 (C) 0.89 (D) 0.96 (E) 1.89




What does TC and TM seperately indicate? Which formula is suit to solve this architect-related problem?

[zc0000]

I know ~ I know how to do this
At first I negelect the knowlage point about "cache miss formula"