`a,` $\dfrac{15}{40}$ `-` $\dfrac{7}{40}$

`=` $\dfrac{15-7}{40}$

`=` $\dfrac{8}{40}$ `=` $\dfrac{1}{5}$

`b,` $\dfrac{31}{42}$ `-` $\dfrac{13}{42}$

`=` $\dfrac{31-13}{42}$

`=` $\dfrac{18}{42}$ `=` $\dfrac{3}{7}$

`c,` $\dfrac{51}{64}$ `-` $\dfrac{19}{64}$

`=` $\dfrac{51-19}{64}$

`=` $\dfrac{32}{64}$ `=` $\dfrac{1}{2}$

`d,` $\dfrac{70}{51}$ `-` $\dfrac{36}{51}$

`=` $\dfrac{70-36}{51}$

`=` $\dfrac{34}{51}$