How many different messages can be transmitted in n microseconds using three different signals... 
How many different messages can be transmitted in n microseconds using three different signals if one signal requires 1 microsecond for transmittal, the other two signals require 2 microseconds each for transmittal, and a signal in a message is followed immediately by the next signal?

I initially got it wrong because I put as the initial condition: $a_0=0, \space a_1=1$
I found this solution online:



Why is the initial condition $a_2=3$, and not $a_2=2$?  It says the other two signals require $2$ microseconds, so I believe $a_2=2$ because in $2$ microseconds we can only send $2$ signals.

i found a online solution in which it is showing An = An-1+An-2 because we have two choise here either we can send first signal which 1 second time or we can send another signal which takes two second time anyone of them we can send first so i think the value of A1 = 1 because in 1st second we can send  a signal which takes 1 second time and A2 = 1 because in 2 second we cannot send two signal which takes 1 second it is cleared by question so in two second we can send only one signal which takes two second.
