HKIMO 2020 Grade 5 National Final (Heat) Set 01 Question 7

Find $\frac1{3\times7}+\frac1{7\times11}+\frac1{11\times15}+\cdots+\frac1{31\times35}+\frac1{35\times39}$. Write the answer in simplest form.

Source glossary (English–Vietnamese): Value: giá trị; Simplest form: dạng tối giản.