Assuming that Richard always reads with the same rate, there is a direct proportion between the time spent reading and the portion of book read:
[tex] \text{portion}_1 : \text{time}_1 = \text{portion}_2 : \text{time}_2 [/tex]
So, we can set up and solve the following proportion:
[tex] \dfrac{1}{4} : \dfrac{4}{5} = x : 1 \iff x = \dfrac{\frac{1}{4}}{\frac{4}{5}} = \dfrac{1}{4}\cdot \dfrac{5}{4} = \dfrac{5}{16} [/tex]
