Answer:
[tex]\large\boxed{t=\dfrac{11}{21}}[/tex]
Step-by-step explanation:
[tex]\dfrac{3}{7}+t=\dfrac{20}{21}\qquad\text{subtract}\ \dfrac{3}{7}\ \text{from both sides}\\\\\dfrac{3}{7}-\dfrac{3}{7}+t=\dfrac{20}{21}-\dfrac{3}{7}\qquad\text{LCD(7, 21)=21}\\\\t=\dfrac{20}{21}-\dfrac{3\cdot3}{7\cdot3}\\\\t=\dfrac{20}{21}-\dfrac{9}{21}\\\\t=\dfrac{20-9}{21}\\\\t=\dfrac{11}{21}[/tex]
