math question

since LCM(20,40)=40, let's express both fractions so that both fractions have a numerator of 40. 39/40=39/40, 7/20=14/40. 39/40-14/40=(39-14)/40=25/40. Divide both the numerator and denominator by 5 and get 5/8. This is 0.675 in decimal expansion.

what is 39/40-7/20

$\begin{array}{|rcll|} \hline && \frac{ 39}{40}-\frac{7}{20}\\ &=& \frac{ 39}{2\cdot 20}-\frac{7}{20}\\ &=& \frac{1}{20}\cdot \frac{ 39}{2}-\frac{1}{20}\cdot 7 \\ &=& \frac{1}{20}\cdot \left( \frac{39}{2} - 7 \right) \\ &=& \frac{1}{20}\cdot \left( \frac{39}{2} - \frac{14}{2} \right) \\ &=& \frac{1}{20}\cdot \left( \frac{25}{2} \right) \\ &=& \frac{25}{20\cdot 2} \\ &=& \frac{5\cdot 5}{4\cdot 5 \cdot 2} \\ &=& \frac{5}{4 \cdot 2} \\ &=& \mathbf{\frac{5}{8}} \\ &=& \mathbf{0.625} \\ \hline \end{array}$