Hi Paula;
Total box seats...2b
Total reserved seats...2r
Total lawn seats...l
FIRST EQUATION...2b+2r+l=3900
SECOND EQUATION...[2b($10)]+[2r($6)]+[l($4)]=$26,820
The units of dollars on both sides cancels...
................................[2b(10)]+[2r(6)]+[l(4)]=26,820
................................20b+12r+4l=26,820
Divide both sides by 4......5b+3r+l=6,705
THIRD EQUATION...[(b)(10)]+[(r)(6)]+[(l)(4)]=16,190
..............................10b+6r+4l=16,190
Divide both sides by 2.....5b+3r+2l=8,095
Let's subtract the SECOND from the THIRD...
THIRD.........5b+3r+2l=8,095
SECOND...-(5b+3r+l=6,705)
....................l=1390
Let's plug l=1390 into all three equations...
FIRST EQUATION...2b+2r+l=3900
............................2b+2r+1390=3900
............................2(b+r)=2510
............................b+r=1255
SECOND EQUATION...5b+3r+l=6705
................................5b+3r+1390=6705
................................5b+3r=5315
THIRD EQUATION...5b+3r+2l=8095
.............................5b+3r+[(2)(1390)]=8095
.............................5b+3r+2780=8095
.............................5b+3r=5315
Let's take the FIRST EQUATION and isolate one variable. I randomly select b...
FIRST EQUATION...b+r=1255
...........................b=1255-r
Let's plug this into the THIRD EQUATION...
THIRD EQUATION...5b+3r=5315
...........................[(5)(1255-r)]+3r=5315
............................6275-5r+3r=5315
............................6275-2r=5315
............................-2r=-960
...........................r=480
FIRST EQUATION...2b+2r+l=3900
...........................2b+[(2)(480)]+1390=3900
...........................2b+960+1390=3900
...........................2b+2350=3900
............................2b=1550
.............................b=775
SECOND EQUATION...5b+3r+l=6705
................................[(5)(775)]+[(3)(480)]+1390=6705
...............................3875+1440+1390=6705
...............................6705=6705
THIRD EQUATION...5b+3r+2l=8095
............................[(5)(775)]+[(3)(480)]+[(2)(1390)]=8095
............................3875+1440+2780=8095
............................8095=8095
TOTAL BOX SEATS...2b...(2)(775)=1550
TOTAL RESERVE SEATS....2r...(2)(480)=960
TOTAL LAWN......l...1390