34+(239-606:6)×4-393:3=34+(239-101)×4-131=34+552-131=455
<span>1,7а + 3,1а = 5,7-2,1
4.8а = 3.6
а= 3,6/4.8
a=0.75</span>
<span>(2 7/12 +1,75):13+2,5:3 3/4=1
1) 2 7/12+1,75= 31/12+175/100=1550/600+1050/600=2600/600=13/3
2) 13/3:13=13/3*1/13=1/3
3) 2,5:3 3/4=25/10:15/4=25/10*4/15=10/15=2/3
4) 1/3+2/3=3/3=1</span>
X+13.1+x=90
2x+13.1=90
2x=90-13.1
2x = 76.9
x=76.9/2
x=38.45