Let's call the time it takes to install all the chairs from start to finish "t". The workers install 840 chairs per day, so in "t" days they will have installed 840 * t chairs.
At the end of 5 days, they still have 240 chairs to install, so they have installed 840 * 5 chairs so far. We can use this information to write an equation:
840 * t - 840 * 5 = 240
Expanding and simplifying the equation gives us:
840t - 4200 = 240
Adding 4200 to both sides gives us:
840t = 4440
Finally, dividing both sides by 840 gives us:
t = 5.2857142857...
So it takes approximately 5.29 days to install all the chairs from start to finish.