Two distinct numbers a , b are chosen from { 1 , 2 , … , 39 } . Find the probability that 7 a − 9 b = 0 .

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MathematicsProbabilityGeometrical ProbabilityMedium2 minQB

MathematicsMediumsingle choice

Two distinct numbers $a, b$ are chosen from ${1, 2, \dots, 39}$ . Find the probability that $7 a - 9 b = 0$ .

Answer:

Solution:

The valid pairs are (9, 7), (18, 14), (27, 21), (36, 28). Hence the probability is $4/ (2 39) = 4/741$ .

Stream:JEESubject:MathematicsTopic:ProbabilitySubtopic:Geometrical Probability

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