
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
a) Helena and Gabriel jump simultaneously from a raft in a river and swim in opposite directions. Gabriel swims downstream with the current, and Helena swims upstream against the current, possibly at a different speed. After 5 minutes, they turn around and swim back to the raft, each maintaining a constant speed the entire time. Who returns to the raft first? (During this time, the raft was anchored, i.e., it remained stationary despite the current.)

b) At his usual speed, a man goes down a river $15 \mathrm{~km}$ long in 5 hours less than the time he takes to swim the same distance upstream. If he doubles his usual speed, he goes down the river using just 1 hour less than the time he takes to swim back. Assuming the river current's speed remains constant during the trips, what is its value in $\mathrm{km} / \mathrm{h}$?

Present the answer in LaTex format: \boxed{Your answer}
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