It takes Susan 3 hours to fix 2 broken cars and it takes Joseph 4 hours to fix 3 broken cars. If they work together how long will it take them to fix 17 broken cars?

Combined rates

Susan = 2 car / 3 hr

= 2/3

Joe = 3 car / 4 hr

= 3/4

add the rates to get the new combined rate as 17 cars in "t" hours.

2/3 + 3/4 = 17/t

Need common denominator

2/3 (4/4) + 3/4 (3/3) = 17/t

8/12 + 9/12 = 17/t

17/12 = 17/t

see that "t" is 12 hours