The value of c could be any odd integer because and odd integer subtracted by an even integer is an odd integer.
The ratio is 4:3:1
let the ratio be 4x:3x:x
according to the question,4x+3x+x=40
8x=40
x=5
therefore amount of gravel=x*4=5*4=20

amount of sand=5*3=15

amount of cement=5*1=5
42 firefighters in total.
16 volunteered for the special rescue team.
n + 16 = 42
or it could be this way:
42 - 16 = n
True, you can even test it out yourself. If the domain (x) is the same then it will be a vertical line. Like if you graphed (1,2) (1,4), (1,6) it will be a vertical line on your graph. I hope this helps!