Problem Solving

If P^2-QR=10, Q^2+PR=10, R^2+PQ=10, and R not= Q , what is the value of P^2+Q^2+R^2?

a) 10
b) 15
c) 20
d) 25
e) 30

OA: C

step 1: Subtract equation 2 from equation 3
step 2: Sum all given equations
step 3: and apply the resultant of step 1 in step 2.

I understand this method. However, how will one know to solve it this way?... subtracting one equation from another, summing all equations and then applying the intermediate result to obtain final result.

What is the concept behind this? Please help.

It has to come with practice. There is never going to be a defined approach for any question. Then everyone would score 800 on the test.

You have to identify ways to get to the answer. Practice and practice more.