P(E)<21
⇒r=n∑8Cr8(21)8−r(21)r<21
⇒r=n∑8Cr8(21)8<21
⇒Cn8+Cn+18+…+C88<128
⇒256−(C08+C18+…+Cn−18)<128
⇒C08+C18+…+Cn−18>128
⇒1+8+28+56+70+C58+…+Cn−18>128
⇒163+C58+…+Cn−18>128
⇒n−1≥4
⇒n≥5
A student appeared in an examination consisting of 8 true-false type questions. The student guesses the answers with equal probability. The smallest value of n, so that the probability of guessing at least n correct answers is less than 21, is :
Held on 27 Jul 2021 · Verified 6 Jul 2026.
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