B4:
15 phút = $\frac{15}{60}$ = $\frac{1}{4}$
B5:
a)3$\frac{2}{5}$+2$\frac{5}{7}$
=$\frac{17}{5}$+$\frac{19}{7}$
=$\frac{214}{35}$
b)5$\frac{1}{3}$+4$\frac{5}{6}$
=$\frac{16}{3}$+$\frac{29}{6}$
=$\frac{61}{6}$
c)2$\frac{3}{8}$+3-4$\frac{1}{6}$
=$\frac{19}{8}$+3-$\frac{25}{6}$
=$\frac{43}{8}$-$\frac{25}{6}$
=$\frac{29}{24}$