21,4 минус 5 целых 1/2 минус (16 целых 2/3 минус 6,6).

1) 16 2/3 - 6,6 = 16 2/3 - 6 6/10 = 50/3 - 33/5 = (50 * 5 - 33 * 3) / 15 = 250/15 - 99/15 = 151/15 = 10 1/15

2) 21,4 - 5 1/2 = 21 4/10 - 5 1/2 = 107/5 - 11/2 = (107 * 2 - 11 * 5) / 10 = 214 / 10 - 55/10 = 159/10 = 15 9/10

3) 15 9/10 - 10 1/15 = 159/10 - 151/15 = (159 * 3 - 151 * 2) / 30 = 477/30 - 302/30 = 175/30 = 35/6 = 5 5/6

ответ 5 5/6