Notice that the denominators are 3 and 7, which are factors of 21.
So express all the fractions with denominator 21:
On Monday [tex] \frac{1}{7} = \frac{1}{7} * \frac{3}{3} = \frac{3}{21} [/tex] are sold
On Tuesday [tex] \frac{1}{3} = \frac{1}{3} * \frac{7}{7} = \frac{7}{21} [/tex] are sold
On Wednesday [tex] \frac{2}{7} = \frac{2}{7} * \frac{3}{3} = \frac{6}{21} [/tex] are sold
by Thursday [tex]\frac{3}{21}+\frac{7}{21}+\frac{6}{21} = \frac{3+7+6}{21}= \frac{16}{21} [/tex] of the tickets was sold.
Answer: [tex]\frac{16}{21}[/tex]