Economics - How price of one good depend on another

Given the opportunity to save your life or econ grade, I must respond.

What do the equations mean? In the case of both manufacturers, if their competitors raise their prices, they will see more demand and if they raise their own prices, their demand will drop significantly. If you add the two equations together, you will see that the total demand = 30 - Px - Py so price increases by either company will result in the overall demand going down.

The second question requires you to make a profit equation for X. X makes (Px-3) dollars per sale, so Total Profit = Dx (Px-3). Stop here until you understand that.

From the equation you provided:
Dx = 15 - 2Px + 7 = 22 - 2Px

Put that into the profit equation:
Profit = (22 - 2Px) * (Px - 3)

Find the maximum of that equation. You can use calculus, a graphing calculator or find the roots using algebra and just know that the answer is between the roots. Since the roots (where profit is zero) are 3 and 11, Px = $7.

Strange. I took Econ 101, Marcoeconomics, and Mircoeconomics, and I don't remember this formula. But then, I graduated from college over 50 years ago.

Hello savior! Thanks for the help! I appreciate it so so so much!

Can I ask one more thing? If X then is like a monopoly who faces a marginal revenue MRb(Qb) = 11 - Qb
then how many cups will he try to sell and to what price? I don't really understand how this is different from the earlier question....