1) 4(7-5х)-(12-20х) =28-20x-12+20x=16
2)<span>2(3х-у)-(5х-2у) = 6x-2y-5x+2y=x
</span><span>3) 6(х+2у)-3(2х+у)=6x+12y-6x-3y=9y
</span><span>4) 5(4х+у)-2(х-у)=20x+5y-2x+2y=18x+7y</span>
1 + 14 = 15
14 + 1 = 15
15 - 1 = 14
15 - 14 = 1
2 + 13 = 15
13 + 2 = 15
15 - 2 = 13
15 - 13 = 2
также: 3+12=15, 4 + 11=15, 5 +10=15, 6 + 9=15, 7 + 8 = 15......
7 + 9 = 16
9 + 7 = 16
16 - 9 = 7
16 - 7 = 9
10 + 6 = 16
6 + 10 = 16
16 - 10 = 6
16 - 6 = 10
11 + 5 = 16
5 + 11 = 16
16 - 5 = 11
16 - 11 = 5
также: 4 +12 = 16, 3 + 13 = 16, 2 + 14 = 16, 1 + 15 = 16