River C : 200 + x
River D: x
200 + x + x = 5,540
combine like terms
200 + 2x = 5,540
subtract 200 from both sides
2x = 5,340
divide both sides by 2 to isolate the variable
x = 2, 670
River C: 200 + x = 200 + 2,670 = 2,870
River D: x = 2,670
2,870 + 2,670 = 5,540
