Dave, Frank, and Kathy can process 1,300 catalog orders per day when all three of them are working. Dave and Frank can do 780 orders per day and Frank and Kathy can do 850 orders per day. How many can each do in a day?

(1) x+y+z=1300

(2) x+y=780 = > x = 780 - y

(3) y+z=850 = > z = 850 - y

Replace x and z with y in function (1)

780 - y + y + 850 - y = 1300

1630 - y = 1300

y = 330

Therefore, x = 780 - y = 780 - 330 = 450

z = 850 - y = 850 - 330 = 520

Dave can do 450 catalog orders per day

Frank can do 330 catalog orders per day

Kathy can do 520 catalog orders per day