S/(V+U)+S/(V-U)=t
V - скорость катера
U - скорость течения
S/20+S/16=4.5
1) <span>2х+у=15
y = 15 - 2x
x = (15 - y)/2
2) </span><span>у+6х=23
y = 23 - 6x
x = (23 - y)/6
3) </span><span>0,9х-2у=18
x = (18 + 2y)/0.9 = 20 + 2 2/9y
y = (0.9x - 18)/2 = 9/20 x - 9
4) </span><span>5у+2,5х=1
y = (1 - 2.5x)/5 = 0.2 - 0.5x
x = (1 - 5y)/2.5 = 0.4 - 2y</span>