The standard deviation for the combined assignment is 5 minutes.
To combine the standard deviations, we use:

With our information, we have:
Uhhh , hehegehhdhdhejrkelelke
The first thing you do is substitute 9 for y in the function, which is,
9 - 4 = 5/6 (x - 2)
Then, you subtract on the left side
5 = 5/6 (x - 2)
Then, you multiply by 6 on both sides
30 = 5 (x - 2)
Then, divide both sides by 5
6 = x - 2
Then, at 2 on both sides to isolate x
& your final answer is
x = 8 (this would be the missing value)
800 * ( 6.45 / 100 ) = 8 * 6.45 = 51.6$ ;