Solve for the two unknowns. 3a + 4b = 12 2a + 2b = 15

Let us take the second equation first for finding the value of a

2a + 2b = 15

2a = 15 - 2b

a = (15/2) - b

Now let us put the value of a found from the second equation in the first equation. We get

3a + 4b = 12

3[ (15/2) - b] + 4b = 12

45/2 - 3b + 4b = 12

b = 12 - (45/2)

= (24 - 45) / 2

= - 21/2

Now putting the value of b in the second equation, we get

2a + 2b = 15

2a + [2 * ( - 21/2) ] = 15

2a - 21 = 15

2a = 15 + 21

2a = 36

a = 36/2

= 18

So from the above solutions we come to know that the value of a is 18 and the value of b is - 21/2 (minus twenty one divided by 2).