Which number sentence is true? A. 7.7 |-9.8| B. |-7.7| |-9.8| C. -7.7 |-9.8| D.

Speed of the first car is 30 mph and that of the second car is 20 mph.

Step-by-step explanation:

Step 1: Let the speed of the first car be x mph. Then the speed of the second car = x - 10 mph. Distance traveled by the second car = 60 miles. Find the time taken by the second car to travel this distance.

Time = Distance/Speed

        = 60/x - 10 (1)

Step 2: Find the time taken by the first car to travel given distance of 90 miles.

Time = 90/x (2)

Step 3: Given that both cars take the same amount of time, equate the 2 equations. (1) = (2)

⇒ 60/x - 10 = 90/x

⇒ 60x = 90(x-10)

⇒ 2x = 3(x - 10)

⇒ 2x = 3x - 30

⇒ -x = -30

∴ x = 30 mph

⇒ x - 10 = 30 - 10 = 20 mph