In a random sample of 80 adults, how many are college graduates?

Since there are 80 adults we don't need to consider non-adults number here. Otherwise, we would have to solve this by 2X2 matrix.
Either adults are college graduates (X) or not (Y)
Stmt 1) X + Y = 80
Y = 3X
these 2 eq can be solved to get the value of X

Stmt 2) Similar way
X + Y = 80
Y = 40 + X
these 2 eqns can also be solved for value of X

Both statements are sufficient, Hence D

Lets say college going graduate is X
and college not going graduate is Y

form question X + Y = 80.
Option 1: Y = 3X
so, X + 3X = 80 leads to X = 20. Therefore option 1 is sufficient to answer the question.
Option 2: Y - X = 40.
so, 2Y = 120, leads to Y = 60 and X = 20. therefore option 2 is sufficient to anser the question.

Therefor from above both the options are individually sufficient to answer the question => "D" is the correct choice.

Total of 80 adults.
Let C be the college graduates.
let N be the non college grads.

What is C?

(1) N = 3C
3C + C = 80, SUFFICIENT.
(2) N = 40 + C
40 + C + C = 80, SUFFICIENT

answer: D

Basically, we need to solve for x + y = 80, so we need to solve for 2 unknowns, either by solving for one of them or for both at the same time.

1) This statement tells us that x + 3x = 80, we have one equation and one unknown, so it's sufficient.
2) This tells us that (y + 40) + y = 80, so again we have 1 unknown and 1 equation, so we can solve it. Sufficient.

So we go with D.

Rephrase the question
College = 80 - Non College
C = 80 - NC

i) In the sample, the number of adults who are not college graduates is 3 times the number who are college graduates

3C + C = 80
4C = 80
C = 20

or

Part 1 3
Part 2 1
Ratio 4

80 = Ratio x 20

Non College = 60
College = 20

Sufficient - Linear equation with one variable

Statement II

NC = C + 40

C = 80 - C + 40
2C = 40
C = 20

Sufficient

We need to determine the number of college graduates in a sample of 80 adults. Because some of the 80 adults are college graduates, while others are not, let’s define two variables:

c = the number of adults who are college graduates

n = the number of adults who are not college graduates

Since there are 80 adults in the random sample, we can create the following equation:

c + n = 80

Statement One Alone:

In the sample, the number of adults who are not college graduates is 3 times the number who are college graduates.

From statement one we can say:

n = 3c

Since n = 3c, we can plug 3c for n into the equation c + n = 80.

c + 3c = 80

4c = 80

c = 20

Thus, there are 20 college graduates. Statement one is sufficient to answer the question. We can eliminate answer choices B, C, and E.

Statement Two Alone:

In the sample, the number of adults who are not college graduates is 40 more than the number who are college graduates.

From statement two, we can say:

n = 40 + c

Since 40 + c = n, we can substitute 40 + c for n into the equation c + n = 80.

c + 40 + c = 80

2c = 40

c = 20

There are 20 college graduates.

Statement two is also sufficient to answer the question.

The answer is D.

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