c) Số thích hợp để điền vào chỗ chấm là:

\(\frac{1}{4}\,\, + \,\,\frac{1}{{12}}\,\, = \,\,\frac{{.....}}{3}\)

a) \(\frac{3}{4}\,\, + \,\,\frac{1}{5}\)

\(\begin{array}{l} = \frac{{15}}{{20}} + \frac{4}{{20}}\\ = \frac{{{\bf{19}}}}{{{\bf{20}}}}\end{array}\)

b) \(\frac{{13}}{6}\,\, + \,\,\frac{{11}}{{15}}\)

\(\begin{array}{l} = \frac{{130}}{{60}} + \frac{{44}}{{60}}\\ = \frac{{{\bf{174}}}}{{{\bf{60}}}}\end{array}\)

c) \(\frac{{27}}{7}\,\, + \,\,\frac{1}{{21}}\)

 \(\begin{array}{l} = \frac{{81}}{{21}} + \frac{1}{{21}}\\ = \frac{{{\bf{82}}}}{{{\bf{21}}}}\end{array}\)

d) \(\frac{8}{9}\,\, - \,\,\frac{5}{6}\)

\( = \,\,\frac{{16}}{{18}}\,\, - \,\,\frac{{15}}{{18}}\)

\( = \,\,\frac{{\bf{1}}}{{{\bf{18}}}}\)

e) \(\frac{{18}}{{11}}\,\, - \,\,\frac{{15}}{{22}}\)

\(\begin{array}{l} = \frac{{36}}{{22}} - \frac{{15}}{{22}}\\ = \frac{{{\bf{21}}}}{{{\bf{22}}}}\end{array}\)

f) \[\frac{{11}}{7}\,\, - \,\,\frac{5}{8}\]

\[\begin{array}{l} = \frac{{88}}{{56}} - \frac{{35}}{{56}}\\ = \frac{{{\bf{53}}}}{{{\bf{56}}}}\end{array}\]