When Carly does 15 push-ups and 10 sit-ups, it takes a total of 35 seconds. In comparison, she needs 57 seconds to do 19 push-ups and 19 sit-ups. How long does it take Carly to do each kind of exercise.
+36916 15 p + 10 s = 35
19 p + 19 s = 57 Simultaneous equations....2 unknowns 2 equations
Mutiply eq 1 by - 1.9
-28.5 p - 19 s = -66.5 add to second equation
-9.5 p = -9.5
p = 1 second
then YOU should be able to calculate how long a sit up takes ! 
