7 yr ago, the ages of A and B were in the ratio 4 : 5 and

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7 yr ago, the ages of A and B were in the ratio 4 : 5 and 7 yr hence, they will be in the ratio 5 : 6. The present age of B is ?

1. 56 yr
2. 63 yr
3. 70 yr
4. 77 yr

Correct Option: D

Let 7 yr ago, ages of A and B 4k yr and 5k yr, respectively.
Then, present age of A = 4k + 7
and present age of B = 5k + 7
Now, according to the question,
∴ (4k + 7 + 7) / (5k + 7 + 7) = 5/6
⇒ 24k + 84 = 25k + 70
⇒ k = 14
Hence, B's present age = 5 x 14 + 7 = 77 yr