IF A+B=15, B+C=20, C+A=25 THEN A+B+C ?

IF A+B=15,

B+C=20,

C+A=25

THEN

A+B+C ?

Hello,

$A+B+0C=15$

$0A+B+C=20$

$A+0B+C=25$

Add all three

$2A+2B+2C=60$

Factor 2

$2(A+B+C)=60$

Divide by 2

$A+B+C=30$

A+B=15      10+5=15

B+C=20       5+15=20

C+A=25      15+10=25

A+B+C = 30  

a+b=15

b+c=20

c+a=25

then add,

a+c=25

b+c=20

a+b=15

2a+2b+2c=60

divide by 2 on both sides and you get

a+b+c=30

just add all equations. You will get 2A+2B+2C=60

so A+B+C=60/2 = 30