Объяснение:
x1+x2+x3+x4= 30
x2+x3+x6+x7= 28
x3+x4+x5+x6= 42
x2+x3= 8
x3+x6= 10
x3+x4= 5
x3 = 3
x1+x2+x3+x4+x5+x6+x7=?
x4 = 5-x3 = 5-3 = 2
x6 = 10-x3 = 10-3 = 7
x2 = 8-x3 = 8-3 = 5
x5 = 42-x3-x4-x6 = 42-3-2-7 = 30
x7 = 28-x2-x3-x6 = 28-5-3-7 = 13
x1 = 30-x2-x3-x4 = 30-5-3-2 = 20
x1+x2+x3+x4+x5+x6+x7 = 20+5+3+2+30+7+13 = 80 (владеют предложенными языками)
100-80 = 20 туристов не владеют предложенными языками
