Question

The sum of a number and 3 times a second number is represented by a+3b= 39. The sum of half the of the first number and 1/3 of the second number is represented by 1/2a+1/3b=9. What is the first number a? What is the second number b?

Answer 1

if we multiply the second equation by 2 we get

a + 2/3b = 18 ............(3)
a + 3b = 39................(1) 
(1) - (3) gives:-
7/3 b = 21
b =  21 * 3/7 = 9 

plug b = 9 in equation (1):-

a + 3(9) = 39 

a = 39-27 = 12 

Answer:- a = 12 and b = 9.