Code:

| | || ............them's the pipes!
@7:20 @4:35 @11:x
| | | | | |
| | | | | |
---- ---- ----
140 140 140

The LCM of 20 and 35 being 140, then assign 140 as the buckets' size.

Easy to see that 1st pipe has "speed" of 7 per minute, 2nd of 4 per minute.

Combining the pipes then means a combined speed of 7+4 = 11 per minute.

We let x = time this will take.

11x = 140

x = 140 / 11 = 12 8/11

May be a good/easy way to at least keep the 7th graders interested,

with you at the blackboard describing the above...

As you can see, a real easy equation comes out.

Perhaps that was the problem's intention?