(-3.9 * 2 4/5 + 2.66) : (-3 1/5) - 2.1 = 77/160,
1) -3.9 * 2 4/5 = -39/10 * 14/5 = - 273/25 = -10 23/25 (или -10,92),
2) -10,92 + 2,66 = -8,26,
3) -8,26 : (-3 1/5) = 826/100 : 16/5 = 826/100 * 5/16 = 826/320 = 2 93/160,
4) 2 93/160 - 2,1 = 413/160 - 21/10 = 413/160 - 336/160 = 77/160