If the actual length of the bridge is 4200 feet, then what is the scale of the diagram of the bridge?

4200/6 = 700 not the answer 
4200/5 = 840 is the answer 
but the bridge measures 1 2 3 4 5 6 so you are trying to say that it should be 6 right but it is 5 
i.e. 1 unit is 840 
you see the scale that is 1 2 3 4 5 6 
840feet is distance between two units i.e. from 1 to 2 its 840, from 2 to 3 its 840, from 3 to 4 its 840, from 4 to 5 its 840 and from 5 to 6 its 840 so count how much 840's have come yeh thats 5... 
So 5 * 840 = 4200..or 840 + 840 + 840 + 840 + 840 = 4200 .. you got that