Question

Y is a random variable that is distributed N(-16, 1.21). Find k such that Prob(-15.043 < Y ≤ k) = 0.1546. (Round your answer with 4 digits after the decimal.)

Answer 1

Transform Y to Z, which is distributed N(0, 1), using the formula

Y = µ + σZ

where µ = -16 and σ = 1.21.

Pr[-15.043 < Y ≤ k] = 0.1546

Pr[(-15.043 + 16)/1.21 < (Y + 16)/1.21 ≤ (k + 16)/1.21] = 0.1546

Pr[0.791 < Z ≤ (k + 16)/1.21] ≈ 0.1546

Pr[Z ≤ (k + 16)/1.21] - Pr[Z < 0.791] = 0.1546

Pr[Z ≤ (k + 16)/1.21] = 0.1546 + Pr[Z < 0.791]

Pr[Z ≤ (k + 16)/1.21] ≈ 0.1546 + 0.786

Pr[Z ≤ (k + 16)/1.21] ≈ 0.940

Take the inverse CDF of both sides (Φ(x) denotes the CDF itself):

(k + 16)/1.21 ≈ Φ⁻¹ (0.940) ≈ 1.556

Solve for k :

k + 16 = 1.21 • 1.556

k ≈ -14.118