Since she has seven key the probability that the first one will open the door is

if it takes three attempts to open the door,the probability is

if the three attempts would not work
then the probability that the fourth attempt will open the door is

hope this helps
Answer:
No solution.
Explanation:
The roots (zeros) are the x values where the graph intersects the x-axis. To find the roots (zeros), replace f(x) with 0 and solve for x.
The mean can be found by adding the two numbers and dividing by 2
let the second number be y
(x + y)/2 = 1/2x + 1
solve for y
multiply each side by 2
x + y = 2(1/2x + 1)
distribute
x + y = x + 2
subtract x on both sides
y = 2
ANSWER: the second number is 2