Let's solve for x in the first equation:

200(x + 71) = 54000
200x + 14200 = 54000
200x = 54000 - 14200
200x = 39800
x = 39800 / 200
x = 199

Now let's solve for c in the second equation:

900(16 - c) = 5400
14400 - 900c = 5400
-900c = 5400 - 14400
-900c = -9000
c = -9000 / -900
c = 10

Finally, let's solve for x in the third equation:

400(x - 12) = 3600
400x - 4800 = 3600
400x = 3600 + 4800
400x = 8400
x = 8400 / 400
x = 21

Therefore, the solutions are:
x = 199
c = 10
x = 21