Question

USE THE SUBSTITUTION METHOD TO SOLVE
3M+N=20
M-8N=15

Answer 1

to use substitution, we need to solve for one of the variables. Im choosing to use the second equation and solve for M because it looks easy, so we obtain:
\[M-8N = 15 \Rightarrow M = 8N+15\]
Now Im going to substitute that value of M into the first equation; everywhere there is an M in the first equation I will replace it with 8N+15:
\[3M+N+20 \Rightarrow 3(8N+15)+N=20 \Rightarrow 24N+45+N=20\]
\[25N=-25 \Rightarrow N=-1\]
now that I have N = -1, i can put that back into the first equation I had:
\[M=8N+15 \Rightarrow M=8(-1)+15 \Rightarrow M = -8+15 \Rightarrow M = 7\]