Question

A one pound brisket takes 50 minutes to cook at 375°F. How long would it take a 3 1/2 pound brisket to cook?

Answer 1

Assuming its the same temp, use this equation for pounds and minutes then solve:

$\frac{1}{3.5} = \frac{50}{x}$

cross multiply 

50(3.5) = 175 minutes 

it'll take an 2 hours 55  minutes

Answer 2

Assuming cooking time is linear, multiplying 50 by 3.5 would give you 175 minutes or two hours and 55 minutes.