Question

A baby wriggled so much that weighing him at the clinic was a problem. So the doctor held the baby and stood on a scale. Then the nurse held the baby and stood on the scale. Then the doctor held the nurse who held the baby and stood on the scale. the three results were 78 kg, 69 kg and 142 kg respectively. What was the weight of the baby.
Long question but help me out ( it was 69 kg I mean)

Answer 1

x = weight of the baby.
y = weight of the doctor.
z = weight of the nurse.

x + y = 78 so y = 78 - x
x + z = 69 so z = 69 - x
x + y + z = 142

substitute y = 78 - x and z = 69 - x into x + y + z = 142

x + y + z = 142
x +78 - x + 69 - x = 142
-x + 147 = 142
-x = - 5
 x = 5

answer

the weight of the baby was 5 kg

Answer 2

Let the weight of the baby be B 
weight of the Nurse be N 
weight of the Doctor be D 

Now if we fit those into equations we have:

1. B+D= 78
2. B+N= 69
3. B+D+N= 142

Now take 1 and 3:

B+D= 78 so substitute 78 in the thrid equation were B+D is
78+N= 142 ( what plus 78 will get you 142)
78+64= 142

The nurse's weight is 64 kg

Now that we have the nurses weight. You take the combined weight of the baby and nurse and subtract 64

The baby weighs 5kg

now take the doctors and babies combine weight and subtract 4

The Doctor weighs 73 kg


ANSWER:

The baby weighs 5kg, the nurse weighs 64, and the doctor weighs 74.

Sorry for the long answer but hope this helps.