What is the value of x in the solution to the following system of equations? x+y=39 and 3x+4y=136

Question

What is the value of x in the solution to the following system of equations?  x+y=39 and 3x+4y=136

anonymous · Answer

x=-y+39 and -4/3y+136/3

anonymous · Answer

x+y=39 and 3x+4y=136
   -y  -y             -4y    -4y
x=38              3x= 132
                        /3   /3 
                       x= 66

anonymous · Answer

ok  thats answer

anonymous · Answer

kjackson

anonymous · Answer

that's not on the answer document

anonymous · Answer

what

anonymous · Answer

well, looking at my answers, the number could be 10,20,29,or30

anonymous · Answer

Take the two equations, realize that you can add and subtract the two equations from each other. What I mean by that is, you can add x + y = 39 (from the first equation) to 3x + 4y = 136 (from the second) to get 4x + 5y = 175. Now some cleverness comes into play. We want to get one of the variables to cancel. To do this we can multiply either equation by a constant. Lets multiply the first equation by 4 and subtract it from the second. 4x + 4y - (3x + 4y) = 156 - 136. When you go through and cancel you boil it down to x = 20.

anonymous · Answer

Oh, okay, that makes sense.

anonymous · Answer

You can also verify your answer if you need to do this on a test. When you arrive at a conclusion, for instance x = 20. Plug 20 in for x in one of the equations and solve for y. From equation 1 we get 20 + y = 39, which we can see y = 19. Now plugging that into equation 2 we get 3(20) + 4(19) = 136 so our answer is correct.

anonymous · Answer

Oh, thanks!  I understand.

anonymous · Answer

mary100 your answer is wrong there is only one answer not to. The correct answer is x=20 it is that answer because you can solve equations by graphing 

x+y=39                                  3x+4y=136                       Put your answer
-x   -x                                   -3x       -3x              in your calc and graph 
y=-x+39                                      4y=-3x+136      it than you will see 20
                                                    divide by 4          than check your 
                                                 y=-3/4x+34          answer

anonymous · Answer

so I can do this with or without a calculator?

anonymous · Answer

You  can do it without a calculator but the calculator just checks it

anonymous · Answer

Coming from someone with a B.S. in Math, you NEVER NEED a calculator, but they are often helpful.

anonymous · Answer

Well, I came from poor schools and back then we never got to use calculators.

anonymous · Answer

If you do solving equations by graphing you need a calculator but if you do elimination or substitution you totally do not need a calculator

anonymous · Answer

thanks, you are great!!