Dưới đây là kết quả tính cho các bài toán trong đề bài:

### Bài 1:

a) \( 2 + \frac{3}{4} = 2.75 \)

b) \( \frac{1}{3} + \frac{3}{2} - \frac{7}{4} = \frac{1}{3} + 1.5 - 1.75 = \frac{1}{3} - 0.25 = \frac{1}{12} \)

c) \( \frac{1}{6} + \frac{3}{8} - \frac{1}{4} = \frac{4}{24} + \frac{9}{24} - \frac{6}{24} = \frac{7}{24} \)

d) \( \frac{5}{12} + \frac{3}{8} \times \frac{4}{9} = \frac{5}{12} + \frac{12}{72} = \frac{5}{12} + \frac{1}{6} = \frac{5}{12} + \frac{2}{12} = \frac{7}{12} \)

e) \( \frac{4}{5} - \frac{1}{5} \times \frac{7}{2} = \frac{4}{5} - \frac{7}{10} = \frac{8}{10} - \frac{7}{10} = \frac{1}{10} \)

f) \( \frac{16}{9} - \frac{4}{15} : \frac{2}{5} = \frac{16}{9} - \frac{4}{15} \times \frac{5}{2} = \frac{16}{9} - \frac{10}{15} = \frac{16}{9} - \frac{2}{3} = \frac{16}{9} - \frac{6}{9} = \frac{10}{9} \)

### Bài 2:

a) \( \frac{7}{9} \times \frac{3}{5} \div \frac{5}{8} = \frac{7}{9} \times \frac{3}{5} \times \frac{8}{5} = \frac{7 \times 3 \times 8}{9 \times 5 \times 5} = \frac{168}{225} = \frac{56}{75} \)

b) \( \frac{3}{4 \times 21} \times 25 \div \frac{3}{5} = \frac{75}{84} \)

c) \( \frac{15}{16} \div \frac{8}{3} \times \frac{3}{4} = \frac{15}{16} \times \frac{3}{8} = \frac{45}{128} \)

d) \( \frac{21}{4} \times \frac{16}{14} \times \frac{8}{3} = 21 \div 3 = 7, 16 \div 8 = 2 => 7 \times 2 = 14 \)

### Bài 3:

a) \( \frac{21}{25} \times \frac{2}{5} + \frac{21}{25} \times \frac{3}{5} = \frac{21}{25} \times 1 = \frac{21}{25} \)

b) \( \frac{3}{4} \times \frac{3}{14} \div 6 = \frac{9}{84} \div 6 = \frac{3}{168} \)

c) \( \frac{3}{10} \times \frac{7}{4} - \frac{3}{10} \times \frac{3}{4} = \frac{21}{40} - \frac{9}{40} = \frac{12}{40} = \frac{3}{10} \)

### Bài 4:

a) \( x \times \frac{3}{7} = 1 - \frac{5}{8} \Rightarrow x = \frac{8}{3} \times \frac{3}{1} = \frac{8 - 7.5}{3} = \frac{1}{3} \)

b) \( \frac{8}{3} - x = \frac{9}{10} + \frac{5}{3} \Rightarrow x = \frac{8}{3} - 3.3 = \frac{8}{3} - \frac{10}{3} = -\frac{2}{3} \)

c) \( x \times \frac{5}{9} = 2 - \frac{11}{4} \Rightarrow x = \frac{2 - 11/4}{5/9} = 0.36 \)

Hy vọng những kết quả trên sẽ giúp ích cho bạn!