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MathRevolution
Forget conventional ways of solving math questions. In PS, IVY approach is the easiest and quickest way to find the answer.


If a real number x is chosen at random in the interval [0,3] and a real number y is chosen at random in the interval [0,4], what is the probability that x < y?

A) 1/2
B) 7/12
C) 5/8
D) 2/3
E) 3/4

Hi! Engr
I'm afraid that you made a mistake to understand the problem because x and y are not integers but real numbers.
So we cannot count the all the cases of (x, y) satisfying x<y.

The following is my approach.... check this out...

Since x is in [0,3] and y is in [0,4] we can put them in the coordinate plane.
That means in the coordinate plane x and y satisfy 0<= x <=3 and 0<= y <=4.
A point (x, y) can be in the rectangle with having (0, 0), (3, 0), (0, 4), (3, 4) as its 4 vertices.
Moreover the points with x<y should be in the trapezoid above the line through (0, 0) and (3, 4).

The area of the trapezoid above the line(joining (0, 0) and (3, 4)) is 15/2 and the area of the rectangle is 12.

So the probability is (15/2)/12 = 15/24=5/8.


The answer is, therefore, (C).


MathRevolution, thank you. That is indeed an incorrect assumption on my end. Also, I think the text in red above is not correct. I believe you wanted to write the line joining (0,0) and (3,3) and NOT (3,4). The line joining (0,0) and (3,4) will not leave a trapezoid at the top.
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MathRevolution
Forget conventional ways of solving math questions. In PS, IVY approach is the easiest and quickest way to find the answer.


If a real number x is chosen at random in the interval [0,3] and a real number y is chosen at random in the interval [0,4], what is the probability that x < y?

A) 1/2
B) 7/12
C) 5/8
D) 2/3
E) 3/4

Hi! Engr
I'm afraid that you made a mistake to understand the problem because x and y are not integers but real numbers.
So we cannot count the all the cases of (x, y) satisfying x<y.

The following is my approach.... check this out...

Since x is in [0,3] and y is in [0,4] we can put them in the coordinate plane.
That means in the coordinate plane x and y satisfy 0<= x <=3 and 0<= y <=4.
A point (x, y) can be in the rectangle with having (0, 0), (3, 0), (0, 4), (3, 4) as its 4 vertices.
Moreover the points with x<y should be in the trapezoid above the line through (0, 0) and (3, 4).

The area of the trapezoid above the line(joining (0, 0) and (3, 4)) is 15/2 and the area of the rectangle is 12.

So the probability is (15/2)/12 = 15/24=5/8.


The answer is, therefore, (C).

Thanks MathRevolution. It was tough one and I couldn't crack it.
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Another way to look at the problem:
Break up the range of y into two parts.
1. \(y>3\)
2. \(0\leq{y}\leq{3}\)

We can consider the probability of \(y>x\) for each part, then add them up.

For part 1, if \(y>3\), then it will always be greater than x no matter what x is. y will be in this range 1/4 of the time. Probability = 1*1/4 = 1/4.
For part 2, both x and y are in the range of [0,3], so we can logically conclude that \(y>x\) half the time (similarly \(x>y\) half the time). y will be in this range 3/4 of the time. 1/2*3/4 = 3/8

Add up both parts:
1/4 + 3/8 = 5/8

Answer C
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That is indeed an incorrect assumption on my end. Also, I think the text in red above is not correct. I believe you wanted to write the line joining (0,0) and (3,3) and NOT (3,4). The line joining (0,0) and (3,4) will not leave a trapezoid at the top.


Hi! Engr
Thank you for pointing out my error. As you pointed out the line is joining (0,0) and (3,3) and NOT (3,4). That makes the diagram a trapezoid.
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Asked: If a real number x is chosen at random in the interval [0,3] and a real number y is chosen at random in the interval [0,4], what is the probability that x < y?

Probability that x<y = (3/4)(1/2) + (1/4)(1) = 3/8 + 1/4 = (3+2)/8 = 5/8

IMO C
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bablu1234
If a real number x is chosen at random in the interval [0,3] and a real number y is chosen at random in the interval [0,4], what is the probability that x < y?

A) 1/2
B) 7/12
C) 5/8
D) 2/3
E) 3/4
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