Solve the pair of linear equations 3p + 5q = 21 and 2p + q = 7 by the substitution method. 

The pair of linear equations is given as: 

3p + 5q = 21 ... (1) 

2p + q = 7 ... (2) 

From equation (2): 

q = 7 − 2p ... (3) 

Substituting the value of q in (1): 

3p + 5(7 - 2p) = 21 

3p + 35 - 10p = 21 

-7p = -14 

p = 2  

Substituting p = 2 in (3): 

q = 7 − 2 (2) = 7 − 4 = 3 

Thus, the solution is p = 2 and q = 3.