probability of sequential events something has to travel from A to B. between these points there are two stages, where at each one it can fail with a probability of 0.3.
what is the total probability that the thing successfully travels from A to B?
 A: If you assume that two stages are independent, then probability of success is multiplication of success of each stage.
Hence $P = 0.7*0.7 = 0.49.$
A: I'll call your "it" a "car" and I'll assume that if the car fails at stage 1, then the car does not reach stage 2. 
Presumably, the probability that the car fails at the second stage given that the car did not fail at the first stage is $0.3$. (Note then  that if $S_1$ is the event "the car fails at stage 1" and $S_2$ is the event "the car fails at stage 2", then $S_1$ and $S_2$ are not independent.)
Let 
$\ \ \ \ \ \ \ \ S_1$ be the event that the car fails at stage 1, 
and let 
$\ \ \ \ \ \ \ \ S_2$ be the event that the car fails at stage 2.
You can use the multiplication rule to find the probability that the car makes the journey: 
$$
P(S_1^C\cap S_2^C)=P(S_1^C)P(S_2^C\mid S_1^C)=(0.7)(0.7)=.49.
$$
Or, you can argue as follows:
Noting that $S_1\cap S_2=\emptyset$ and that $S_2\subset S_1^C$, the probability that the journey is unsuccessful is
$$\eqalign{
P(S_1 \cup S_2)&=P(S_1)+P(S_2)\cr
&=P(S_1)+P(S_2\cap S_1^C )\cr
&= P(S_1)+P(S_2|S_1^C )P(S_1^C)\cr
&=(0.3)+(0.3)(0.7)\cr
&=0.51.
}
$$
So the probability that the car makes the journey  is $1-0.51=0.49$.
