A student studies for X number of hours during a randomly selected school day. The probability that X can…

A student studies for X number of hours during a randomly selected school day. The probability that X can take the values, has the following form, where k is some constant. $P(X=x)= \begin{cases}0 \cdot 2, & \text { if } x=0 \\ k x, & \text { if } x=1 \text { or } 2 \\ k(6-x), & \text { if } x=3 \text { or } 4 \\ 0, & \text { otherwise }\end{cases}$ The probability that the student studies for at most two hours is
  1. 0.1
  2. 0.5
  3. 0.3
  4. 0.7

Solution

Probability of studying at most two hours

The distribution must satisfy $\sum P(X = x) = 1$, where $P(X=0) = 0.2$, $P(X=1) = k$, $P(X=2) = 2k$, $P(X=3) = k(6-3) = 3k$, and $P(X=4) = k(6-4) = 2k$.

Summing yields $0.2 + k + 2k + 3k + 2k = 0.2 + 8k = 1$, so $8k = 0.8$ and $k = 0.1$.

The probability $P(X \le 2) = P(X=0) + P(X=1) + P(X=2) = 0.2 + 0.1 + 0.2 = 0.5$.

$\boxed{0.5}$

Asked in: MHT CET 2025 (05 May Shift 2)

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