Answer:
The probability that the expression Preeti wrote has the value 7 cannot be easily determined.
Step-by-step explanation:
Given that Preeti writes
111111111111111,
then randomly writes + and - between each pair of consecutive 1's.
It is not specified that the + and - signs are written to be alternating, they are said to be just 'random'. So, there are a lot of possibilities. Things like:
1+1-1+1+1-1+1-1-1-1+1+1-1+1-1 = 1
Or
1-1-1+1+1-1-1+1+1-1+1-1+1+1-1 = -1
Or
1-1+1-1-1+1-1+1+1-1-1+1-1+1+1 = 0
And so on. There is a lot of possibilities.
So, clearly, the probability is difficult to determine, as the expression Preeti wrote could have had the value 7 in many different forms.
Because:
1+1+1+1+1+1+1-1+1-1+1-1+1-1+1 = 7
1+1+1+1-1-1-1-1-1+1-1-1-1+1+1 = 7
And many more.
Therefore, the probability that the expression Preeti wrote has the value 7 cannot be easily determined.
.
, 
, and 
. So lets apply the property to simplify our expression:
