b1
Ta có 30abc : abc = 241
(30000+abc): abc = 241
30000 : abc +1 = 241
30000 : abc = 241 - 1
30000 : abc = 240
abc = 30000 : 240
abc = 125
vậy abc là 125
b2
$\frac{1}{2}$+$\frac{1}{6}$+$\frac{1}{12}$+$\frac{1}{20}$+$\frac{1}{42}$
=1-$\frac{1}{1x2}$+$\frac{1}{2x3}$+$\frac{1}{3x4}$+$\frac{1}{4x5}$+$\frac{1}{6x7}$
=1-$\frac{1}{2}$+$\frac{1}{2}$-$\frac{1}{3}$+$\frac{1}{3}$-$\frac{1}{4}$+$\frac{1}{4}$-$\frac{1}{5}$+$\frac{1}{6}$-$\frac{1}{7}$
=1-$\frac{1}{5}$+$\frac{1}{6}$-$\frac{1}{7}$
=$\frac{173}{210}$