ANSWER
1.56 hours
EXPLANATION
If Kim reads 1 chapter in 7 hours, in 1 hour she would read,
[tex]\frac{1chapter}{7h}=\frac{1}{7}chapter/hour[/tex]
Similarly, if Harding reads the same chapter in 2 hours, then in 1 hour he will read,
[tex]\frac{1chapter}{2h}=\frac{1}{2}chapter/hour[/tex]
If they work together, in 1 hour they would read,
[tex]\frac{1}{7}chapter+\frac{1}{2}chapter=\frac{2+7}{2\cdot7}chapter=\frac{9}{14}chapter[/tex]
If they read together 9/14 of a chapter in 1 hour, then they will read 1 full chapter in,
[tex]1chapter\cdot\frac{1h}{\frac{9}{14}chapter}=\frac{14}{9}h\approx1.56h[/tex]
Hence, if they work together, they will read the chapter in 1.56 hours.
