Question

Show that (20,15) is a solution to the system of equations.
5x-2y=70
3x+4y=120

Answer 1

`5\cdot20-2\cdot15=70\\ 100-30=70\\ 70=70\leftarrow \text{ true}\\\\ 3\cdot20+4\cdot15=120\\ 60+60=120\\ 120=120\leftarrow \text{ true}`

Answer 2

{5x-2y=70 /*2

{3x+4y=120

___________________

{10x-4y=140

{3x+4y=120 (+)

13x=260 /:13

x=20

3x+4y=120

3*20+4y=120

4y=120-60

4y=60 /:4

y=15

So the solution is (20; 15)