Question

If 2+3=3, 4+4=5, 4+5=7, 5+7=10, 5+8=12, 6+5=8
then (3+7)+2 = ?

Answer 1

4+4=5
So
2x4=5
4=5/2
Substituting in
4+5=7
(5/2)+5=7
5=14/3

Now
2+3+5+7=3+10
2+3+7=13-(14/3)
2+3+7=25/3=(3+7)+2

Answer 2

Let me replace the left side with variable and right side remain number so
2+3=3 => a+b = 3
4+4=5 => c+c = 5 i.e. c=2.5
4+5=7 => c+d = 7 i.e. d=4.5
5+7=10 => d+e = 10 i.e. e=5.5
5+8=12 => d+f = 12 i.e. f=7.5
6+5=8 => g+d=8 i.e. g=3.5

Lets see what is asked
(3+7)+2 i.e.
(b+e)+a i.e.
(a+b) + e
3 + 7.5
=> 10.5

Answer: 10.5

Comments

How did you get d ?
Did you subtract c from 7 ?
That shouldn't yield you the right answer !!!

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That's right d = 7 - c and in previous step we got c=2.5 so d is 4.5

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The numerical value of c and 7 there are different in meaning in this question, hence can't be subtracted !