Original practice · fully worked
Original variant: conditional interval under an increasing density A quality score X has density f(x)=x/8 for 0<x<4. Given that X exceeds 1, determine the probability that X is less than 2.
A 0.125B 0.1875C 0.250D 0.200E 0.750Variant answer in brief The CDF is x²/16. The mass from one to two is 3/16 and the mass above one is 15/16, giving 1/5, choice D.
Setup
Setup Integrate the density from zero to x to obtain the cumulative distribution F(x)=x²/16.
F ( x ) = ∫ 0 x t / 8 d t = x 2 / 16 F(x)=\int_0^x t/8\,dt=x^2/16 F ( x ) = ∫ 0 x t /8 d t = x 2 /16 Model
Model Within the condition X>1, the target event is the interval from 1 to 2; divide that interval's mass by the mass above 1.
Pr ( X < 2 ∣ X > 1 ) = F ( 2 ) − F ( 1 ) 1 − F ( 1 ) \Pr(X<2\mid X>1)=\frac{F(2)-F(1)}{1-F(1)} Pr ( X < 2 ∣ X > 1 ) = 1 − F ( 1 ) F ( 2 ) − F ( 1 ) Compute
Compute The numerator is 3/16 and the denominator is 15/16, so the conditional probability is 3/15=0.20.
4 / 16 − 1 / 16 1 − 1 / 16 = 3 15 = 0.20 \frac{4/16-1/16}{1-1/16}=\frac3{15}=0.20 1 − 1/16 4/16 − 1/16 = 15 3 = 0.20 Answer
Answer Thus Pr(X<2 given X>1)=0.20, which is choice D.
0.20 (D) \boxed{0.20\quad\text{(D)}} 0.20 (D) ✓ Variant verification record
Computation passed
Computed value 0.2
Main method Integrate the density and form a conditional interval ratio.
Check value 0.2
Check method Direct integrals over (1,2) and (1,4) are 3/16 and 15/16.
Answer key D
Agreement Pass Recorded 2026-07-31T00:00:00Z