71:x=20+7
a+500=4500:5
(1604-y)-108=800
350-(45+a)=60
5t+30-2t=42

71 : х = 20 + 7
71 : х = 27
х = 71 : 27
х = 71/27 = 2 17/27
а + 500 = 4500 : 5
а + 500 = 900
а = 900 - 500
а = 400
(1604 - у) - 108 = 800
1604 - у = 908
у = 1604 - 908
у = 696
350 - (45 + а) = 60
45 + а = 350 - 60
45 + а = 290
а = 290 - 45
а = 245
5t + 30 - 2t = 42
3t + 30 = 42
3t = 42 - 30
3t = 12
t = 12 : 3
t = 4