Question 3: A works 2 hours less than B, so the total working time of A is 10 hour, so A processes 120 parts per hour.
Question 4: Suppose the prices of computer models A and B are 8x and 7x respectively, then after the price changes, A is 8x*( 1-75%), and B is 7x+200. If the changed price ratio is 5: 6, x= 1000 is obtained, then the current prices are 6000 and 7200.