1) 1,7 - 2 1/3 = 17/10 - 7/3 = ( 51 - 70) / 30 = ( - 19/30 )
2) 4,2 - ( - 19/30) = 42/10 + 19/30 = ( 126 + 19)/30 = 145/30
3) - 2,8 - 145/30 = - ( 28/10 + 145/30) = - ( 84 + 145) / 30 = ( - 229/30)
4) - 229/30 + 1 5/6 = - 229/30 + 11/6 = - 229/30 + 55/30 = - 174/30 = - 5,8
ответ - 5,8