How do you solve x=y+4 and 5x+3y=−20 using substitution?

1 Answer
Sarah C
Mar 20, 2016

y=−5
x=−1

Explanation:

First we substitute in our value for x (which is y+4) into the other equation so that we only have one unknown, y.
Then you expand the bracket and simplify it to find y.

5x+3y=−20
5(y+4)+3y=−20
5y+20+3y=−20
8y+20=−20)
8y=−40
y=−408
y=−5

Now we use our value for y in the original equation to find x.

x=y+4
x=−5+4
x=−1

We can then check our answer by substituting both values into the other equation:

(5×−1)+(3×−5)=−20
−5+(−15)=−20)
−5−15=−20
−20=−20
So we know it's right!
It's really important to check your answers, as in a test, it could save you a whole load of marks despite only taking a few seconds:)

Hope this helps; let me know if I can do anything else:)

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