# Time average for two caches , cpu and ram

I have a CPU, with two caches, L1 and L2, with access time T1 and T2 respectively. and The time to access RAM is Tm. The hit ratio for L1 is a, and the hit ratio for L2 is b.

Is the mathematical type of time average this or not?:

Taverage = aT1 + (1-a)T2 + (1-a-b)Tm

(T1 , T2 << Tm)

$$T_{average} = T1 + (1-a)T2 + (1-a)(1-b)Tm$$