If a pipe can fill a tank in x hours, what fractional
part of the tank will be filled in one hour?
Let's look at a bunch of particular cases when we have a
number instead of the letter "x":
Case 1:
If a pipe can fill a tank in 
hour, what fractional
part of the tank will be filled in one hour? Answer: 
th of it.
[That's 1, or 100%, or all of it]
Case 2:
If a pipe can fill a tank in 
hours, what fractional
part of the tank will be filled in one hour? Answer: 
of it.
Case 3:
If a pipe can fill a tank in 
hours, what fractional
part of the tank will be filled in one hour? Answer: 
rd of it.
Case 4:
If a pipe can fill a tank in 
hours, what fractional
part of the tank will be filled in one hour? Answer: 
th of it.
Case 5:
If a pipe can fill a tank in 
hours, what fractional
part of the tank will be filled in one hour? Answer: 
th of it.
Case 6:
If a pipe can fill a tank in 
hours, what fractional
part of the tank will be filled in one hour? Answer: 
th of it.
Case 7:
If a pipe can fill a tank in 
hours, what fractional
part of the tank will be filled in one hour? Answer: 
th of it.
...
Now that list can go on forever and ever. But look at what
is always true in every case above:
If a pipe can fill a tank in a certain number of hours, what
fractional part of the tank will be filled in one hour?
The answer is always 
ths of it.
So if we replace the words "that certain number" by the letter
"x" you could say this about every one of those:
Case x:
If a pipe can fill a tank in 
hours, what fractional part
of the tank will be filled in one hour? Answer: 
ths of it.
So the answer is .
Edwin
