b) 17+ (4-1)=56 21 -t 18 45 15

Решим уравнение:

$$1\frac{7}{18} + (4\frac{21}{45} - t) = 5\frac{6}{15}$$

$$1\frac{7}{18} + (4\frac{7}{15} - t) = 5\frac{2}{5}$$

$$1\frac{7}{18} + 4\frac{7}{15} - t = 5\frac{2}{5}$$

$$5\frac{7}{18} + \frac{7}{15} - t = 5\frac{2}{5}$$

$$5\frac{7\cdot 5}{18\cdot 5} + \frac{7\cdot 6}{15\cdot 6} - t = 5\frac{2\cdot 18}{5\cdot 18}$$

$$5\frac{35}{90} + \frac{42}{90} - t = 5\frac{36}{90}$$

$$5\frac{77}{90} - t = 5\frac{36}{90}$$

$$t = 5\frac{77}{90} - 5\frac{36}{90}$$

$$t = (5-5) + (\frac{77}{90} - \frac{36}{90})$$

$$t = \frac{41}{90}$$

Ответ: $$t = \frac{41}{90}$$