ANSWER
Marty filled 8 buckets of water
Step-by-step explanation
Let
• x: number of buckets Marty filled
,
• y: number of buckets Debby filled
,
• z: number of buckets Melissa filled
We know that Debby filled 10 times as many buckets as Marty:

And that Melissa filled 6 times as many buckets as Marty:

All three of them together fulled 136 buckets:

Replace y and z as functions of x:

And solve for x. First add like terms:

And divide both sides by 17:

We found that Marty filled 8 buckets of water.