Here's the explanation;

For 40 pages

t[a] - t[b] = 4 min --> t[b] = t[a] + 4 ----- (1)

For 50 pages A and B take 6 min --> For 40 pages A and B takes 6/50*40 = 24/5 minutes

Now the equation when both A and B work together to finish a task is : t[a]*t[b]/(t[a] + t[b]) ----- (2)

Putting t[b] = t[a] + 4 from (1) equation (2) becomes:

5t[a]^2 - 68t[a] + 96 = 0 --> (5t[a]-8)(t[a]-12) = 0

---> t[a] = 12 (I discarded t[a] = 8/5 looking at the answer but not sure why it has to be excluded)

So, for 40 pages A takes 12 minutes ---> for 80 pages it will take 24 minutes!

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kris