3. In this case, we are required to compute the amount by which demand would change when the price of another good changes. Let's recall the general version of cross price elasticity:

   From the question, we know that = -2. Therefore, the goods are strongly complementary. Specifically, we know that %QA = -2 x %PB. In other words, since the cross price elasticity is -2, the change in quantity must be twice as large in percentage terms (in the opposite direction) as the change in the price of the other good. So, if the price of good B increases by 5%, the quantity demanded of good A will fall by 10%.

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