Explanation:

Correct option 2:  2

In some code, letters P, Q, R, S, T represent numbers 4, 5, 10, 12, 15. 

It is not known which letter represents which number. 

Here, Q - S = 2S So, Q = 3S …….(I) 

The only possible values of S and Q are: If S = 4, 

then Q = 12 Or, 

if S = 5, then Q = 15 

Also, T = R + S + 3 …….(ii) 

Case 1: If S = 4 and Q = 12 

T = R + S + 3 = R + 7 

This is only possible if T = 12 and R = 5. 

But both Q and T cannot be 12. 

So, we can eliminate this case. 

Case 2: If S = 5 and Q = 15 

T = R + S + 3 = R + 8 

This is only possible if T = 12 and R = 4. 

So, we get R = 4, S = 5, T = 12 and Q = 15. 

It means P must be 10. So, P + R – T = 10 + 4 – 12 = 2