Question 1126323
Ann, Bill, Cara, and Denis weighed in pairs.
 Ann and Bill weigh 180 pounds altogether.
a + b = 180
 Bill and Cara weigh 190 pounds altogether
b + c = 190
 and Cara and Denis weigh 210 pounds altogether.
c + d = 210
 How much do Ann and Denis weigh altogether? 
:
find their total weight.
a + b + c + d = ___
 Replace a+b with 180, replace c+d with 210
180 + 210 = 390 is the total weight
Subtract b + c which is 190
390 - 190 = 200 is the weight of a + d (Ann & Denis)