Question

Out of all the 2-digit integers between 1 and 100, a 2-digit number has to be selected at random. What is the probability that the selected number is not divisible by 7?

Choose one option
Official answer and solution
Official answer
D
From
Official GATE 2013 answer key

Solution

Count the total number of two-digit numbers and the subset divisible by 7, then compute the complementary probability.

  1. Determine total number of 2-digit integers:
    The 2-digit integers between 1 and 100 are 10,11,12,…,9910, 11, 12, \dots, 99.

    Total 2-digit integers=99−10+1=90\text{Total 2-digit integers} = 99 - 10 + 1 = 90
  2. Count 2-digit integers divisible by 7:
    The smallest 2-digit multiple of 7 is 7×2=147 \times 2 = 14.
    The largest 2-digit multiple of 7 is 7×14=987 \times 14 = 98.

    Number of multiples of 7=14−2+1=13\text{Number of multiples of 7} = 14 - 2 + 1 = 13
  3. Count integers not divisible by 7:

    Favorable outcomes=90−13=77\text{Favorable outcomes} = 90 - 13 = 77
  4. Calculate probability:

    P(not divisible by 7)=7790P(\text{not divisible by 7}) = \dfrac{77}{90}

This matches option (D).

Final answer

D

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper
GATE 2013 Computer Science and Information Technology (CS)
Question
62 of 65, printed as GA Q.62
Section
General Aptitude

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