20 Show that $3 \frac{4}{7}-1 \frac{5}{8}=1 \frac{53}{56}$

Step 1 ：Convert the mixed numbers into improper fractions: $\frac{25}{7} - \frac{13}{8}$

Step 2 ：Subtract the two improper fractions and simplify: $\frac{25}{7} - \frac{13}{8} = \frac{109}{56}$

Step 3 ：Convert the result back into a mixed number: $\boxed{1 \frac{53}{56}}$