17÷240 is the same as 17 divided by 10 then by 8 and then by 3, because 10*8*3=240.
To divide 17 by 10 we just move the decimal point: 17/10=1.7.
Now divide by 8: 1.7/8=0.2125; then by 3: 0.2125/3=0.07083333...
(You can divide by 8 by dividing by 2 three times: 1.7/2=0.85, then by 2=0.425 then by 2 again: 0.2125.)