A system of equations is shown below:

Equation A: 4c = d − 8
Equation B: c = 5d 8

Which of the following steps should be performed to eliminate variable d first?

Multiply equation B by 4.
Multiply equation A by −5.
Multiply equation A by 4.
Multiply equation B by 5.

Question

A system of equations is shown below:

Equation A: 4c = d − 8
Equation B: c = 5d   8

Which of the following steps should be performed to eliminate variable d first?

Multiply equation B by 4.
 Multiply equation A by −5.
 Multiply equation A by 4.
 Multiply equation B by 5.

Mercury · Answer

thoughts?

if we want to eliminate variable d, then both equations should have either
- the same coefficient (so you can subtract the two equations and eliminate all d terms)
or
- the same coefficient but with opposite signs (ex: 2d and -2d) so you can add the two equations and eliminate all d terms

now, considering that equation A has d, and equation B has 5d, which of the choices do you think will fulfill one of the above conditions? try seeing what happens when you go through each choice and apply the multiplication stated.