Copy and complete each problem mentally.  

1. $\frac { 5 } { 7 } \cdot \frac { 7 } { 5 } =\boxed {\phantom{a}}$ 

   Answer (a):

   $1$

2. $\frac{8}{13}\cdot\boxed{\frac{\phantom{a}}{\ }}=1$ 

   Hint (b):

   Look at part (a). What do you notice?

   Answer (b):

   $\frac{13}{8}$

3. $\boxed{\frac{\phantom{a}}{\ }}\cdot\frac{31}{27}=1$ 

   Hint (c):

   See part (b).

4. $\frac { 5 } { \boxed {\phantom{a}} } \cdot \frac { 2 } { \boxed {\phantom{a}} } = 1$ 

   Hint (d):

   Look for patterns in parts (a) - (c).