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Question: If the integer a and b are chosen at random between 1 and 100, then the probability that 7<sup>a</su...

If the integer a and b are chosen at random between 1 and 100, then the probability that 7a + 7b is divisible by 5, is

A

14\frac { 1 } { 4 }

B

18\frac { 1 } { 8 }

C

17\frac { 1 } { 7 }

D

149\frac { 1 } { 49 }

Answer

18\frac { 1 } { 8 }

Explanation

Solution

7a always ends with 7, 9, 3, 1.

\7a + 7b is divisible by 5

̃ Favourable events = (25 × 25) × 2

And sample space = 100 × 100

Required probability = 2(25×25)100×100\frac { 2 ( 25 \times 25 ) } { 100 \times 100 }= 18\frac { 1 } { 8 }