Some milk was added to each of the 5 cans. If standard deviation of th...
Step 1 & 2 – Understand the question and draw inferences from the question statement.
To Find: We have to find the standard deviation of the volume of milk in 5 cans after some was added to each of the cans.
Given: Standard Deviation of the 5 cans of milk in the beginning is 3.5 liters.
Let the quantity of milk in 5 cans be a, b, c, d and e.
Finding standard deviation of {a, b, c, d, e}
Step 1
Mean of {a,b,c, d, e} = {a + b + c + d + e}/5 = M
Step 2
Distances of each point from the mean:
Step 3
Squared Distance from Mean
- = (a -M)2 + (b-M)2 + (c -M)2 + (d -M)2 + (e -M)2
Step 4
Average = {(a -M)2 + (b-M)2 + (c -M)2 + (d -M)2 + (e -M)2} /5
Step 5
- Standard Deviation = √ [{(a -M)2 + (b-M)2 + (c -M)2 + (d -M)2 + (e -M)2} / 5] = 3.5
Step 3 – Analyze Statement 1 Independently
Statement 1 – In each can, 20% of the existing quantity of milk was added
The amount of milk be a+20%a, b+20%b, c+20%c, d+20%d, e+20%e
Remember: If all the elements of a set are increased by the x%, the standard deviation of the set also increases by the x%.
So the standard deviation of this set will increase by 20%
New S = 3.5 + 20% *3.5 = Unique number
Alternatively you can calculate the standard deviation of this set as shown below by following same step by step method.
Set = { a+20%a, b+20%b, c+20%c, d+20%d, e+20%e }
Step 1
- Sum of {a+20%a, b+20%b, c+20%c, d+20%d, e+20%e}
- = (1.2)(a + b + c + d + e)
- Mean of {a+20%a, b+20%b, c+20%c, d+20%d, e+20%e}
- = (1.2)(a + b + c + d + e)/5 = 1.2 M
Step 2
Distances of each point from the mean:
- 1.2 (a –M)
- 1.2 (b-M)
- 1.2(c-M)
- 1.2(d-M)
- 1.2(e-M)
Step 3
Squared Distance from Mean
- = 1.22 [(a -M)2 + (b-M)2 + (c -M)2 + (d -M)2 + (e -M)2 ]
Step 4
- Average = 1.22{(a -M)2 + (b-M)2 + (c -M)2 + (d -M)2 + (e -M)2} /5
Step 5
- Standard Deviation = 1.2 √ [{(a -M)2 + (b-M)2 + (c -M)2 + (d -M)2 + (e -M)2} / 5]
- = 1.2 *3.5 = 3.5 + 20% *3.5
- = Unique value
Statement (1) is independently sufficient to arrive at a unique answer.
Step 4 – Analyze Statement 2 Independently
Statement 2 -The average quantity of milk in the cans at the end was 25 liters
We do not know how the quantity changed in each of the cans.
Thus, the information given in statement (2) is not sufficient to arrive at a unique solution.
Step 5 – Analyze Both Statements Together
We get a unique value in Step 3.
Statement (1) independently is sufficient to arrive at a unique answer
Answer: Option (A)