Without values for the other variables, I assume you're asking for a literal equation.
12y+d=-19y+t
31y+d=t
31y=-d+t
y=(-d+t)/31
Answer:

--- Variance
Step-by-step explanation:
Given

Solving (a): Calculate the mean.
The given data is a grouped data. So, first we calculate the class midpoint (x)
For 51 - 58.

For 59 - 66

For 67 - 74

For 75 - 82

For 83 - 90

So, the table becomes:

The mean is then calculated as:



-- approximated
Solving (b): The sample variance:
This is calculated as:

So, we have:


-- approximated
Answer:
D) 3.28
Step-by-step explanation:
820/x=100/2.5
(820/x)*x=(100/2.5)*x - we multiply both sides of the equation by x
820=40*x - we divide both sides of the equation by (40) to get x
820/40=x
20.5=x
x=20.5
now we have:
2.5% of 820=20.5
Answer:
723 students participated in the event
Explanations:
The total number of students in Hanley's school = 1000
0.723 of the students participated in an event
Number of students that participated in the event = 0.723 x 1000
Number of students that participated in the event = 723