X+y=20
x=-4+y how do i solve this equation by using substitution

The solutions is (8,12).

Step-by-step explanation:

The second equation is solved for x so we can replace the first x in the first equation with what the second x in the second equation equals.

That is we can do this:

x+y=20 where x=-4+y

(-4+y)+y=20

-4+y+y=20

Combine like terms:

-4+2y=20

Add 4 on both sides:

    2y=24

Divide both sides by 2:

      y=24/2=12

So if x=-4+y and y=12, then x=-4+12=8.

The solution is (8,12).

This means that (8,12) is a common point on both the given equations.

This means it will make both equations true when plugging it in.

x+y=20 with (x,y)=(8,12):

8+12=20 is true since 8+12 is 20.

x=-4+y with (x,y)=(8,12):

8=-4+12 is true since -4+12=8.

step-by-step explanation:

12: 00 p.m. to 7: 00 p.m. is a total of 7 hours.

4: 00 p.m. to 7: 00 p.m. is a total of 3 hours.

this means that from 4: 00 p.m to 7: 00 p.m. this represents 3/7 of what was lost from 12: 00 p.m. to 7: 00 p.m.

after knowing this, after doing 11,600 - 6,800 = 4,800

4,800 represents 3/7 of what was lost for 7 hours.

we then divide 4,800 by 3 which gets 1,600.

because we know that 1,600 represents 1/7 of how much water was lost, we just have to multiply by 7, which gets 11,200.   after adding 11,200 to how much water was left at 7: 00 p.m. which was 6,800. 11,200 + 6,800 = 18,000

this means that the pool originally had 18,000 gallons in the pool.

you can also check your work by doing 18,000 - 4(1600), which becomes           1800 - 6400 which gets you 11,600.