Calculating fractions of accesses from various levels of memory? (L2 - Main)
The order of accesses is L1 &rightarrow; L2 (&rightarrow; L3) &rightarrow; main memory. The chance of missing is $1 - p$ where $p$ is the hit chance.

In order for an access to hit L2 cache, it must have missed the L1 cache, and hit the L2 cache. So the chance is $(1 - 0.5) \cdot 0.7 = 0.35$.

In order for an access to hit main memory, it must have missed the L1 cache, missed L2 cache and hit main memory. So the chance is $(1 - 0.5)\cdot (1 - 0.7) \cdot 1 = 0.15$.