Đúng ghi Đ, sai ghi S

a) \(\frac{{12}}{5}\,\, + \,\,\frac{4}{7}\,\, = \,\,\frac{{104}}{{35}}\)	c) \(\frac{6}{5}\,\,:\,\,\frac{8}{9}\,\, = \,\,\frac{6}{5}\,\, \times \,\,\frac{8}{9}\,\, = \,\,\frac{{48}}{{45}}\)
b) \(\frac{1}{2}\,\, + \,\,\frac{3}{2}\,\, \times \,\,\frac{{10}}{7}\,\, = \,\,\frac{4}{2}\,\, \times \,\,\frac{{10}}{7}\,\, = \,\,\frac{{20}}{7}\)	d) \(\frac{4}{3}\,\, \times \,\,\frac{6}{8}\,\,:\,\,\frac{7}{9}\,\, = \,\,1\,\, \times \,\,\frac{9}{7}\,\, = \,\,\frac{9}{7}\)

b) Đáp án đúng là: C

Lan thu gom được số ki – lô – gam giấy vụn là:

\(\frac{3}{2} - \frac{1}{4} = \frac{5}{4}\) (kg)

Đức thu gom được số ki – lô – gam giấy vụn là:

\(\frac{5}{4} + \frac{3}{4} = 2\)(kg)

Cả ba bạn thu gom được số ki – lô – gam giấy vụn là:

\(\frac{3}{2} + \frac{5}{4} + 2 = \frac{{19}}{4}\)(kg)

Đáp số: \(\frac{{19}}{4}\) kg giấy vụn

\[\frac{1}{2}\,\, \times \,\,\frac{4}{{13}}\,\, = \frac{{\cancel{2} \times 2}}{{\cancel{2} \times 13}} = \frac{2}{{13}}\] (Ơ)                       \(4\,\, - \,\,\frac{8}{3}\,\, = \,\frac{{12}}{3} - \frac{8}{3} = \frac{4}{3}\)(Ầ)

\(\frac{3}{2}\,\,:\,\,\frac{6}{5}\,\, = \,\,\frac{3}{2} \times \frac{5}{6} = \frac{{3 \times 5}}{{2 \times 2 \times 3}} = \frac{5}{4}\) (M)                   \(\frac{1}{8}\,\, + \,\,\frac{1}{4}\,\, = \,\,\frac{1}{8} + \frac{2}{8} = \frac{3}{8}\)(S)

\(\frac{{16}}{{21}}\,\, \times \,\,\left( {\frac{5}{8}\,\, + \,\,\frac{1}{4}} \right)\,\, = \,\,\frac{{16}}{{21}} \times \,\,\left( {\frac{5}{8}\,\, + \,\,\frac{2}{8}} \right) = \frac{{16}}{{21}} \times \frac{7}{8} = \frac{2}{3}\,\) (N)