Think: "1 chapter in 6 days." Now if we consider 24 days (which is 4 times 6 days), then the number of chapters that can be written in 24 days is 4.
Alternatively, set up and solve an equation of ratios:
1 6 days
-- = -------------
x 24 days
This yields x = 4; 4 chapters can be written in 24 days.
