Ans R

QR ans RS

QR ans RS

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+3 votes

$P, Q, R, \text{and } S$ crossed a lake in a boat that can hold a maximum of two persons, with only one set of oars. The following additional facts are available.

- The boat held two persons on each of the three forward trips across the lake and one person on each of the two return trips.
- $P$ is unable to row when someone else is in the boat.
- $Q$ is unable to row with anyone else except $R$.
- Each person rowed for at least one trip.
- Only one person can row during a trip.

Who rowed twice?

- $P$
- $Q$
- $R$
- $S$

+1 vote

Best answer

- $P$ is unable to row when someone else is in the boat.
- Each person rowed for at least one trip.

These two statements mean that $P$** must return at least once**. So, one return trip is by $P$ alone.

- $Q$ is unable to row with anyone else except $R.$
- This also means that $Q$
**cannot come back**since- if $P$ comes back in first return, and $Q$ comes back in second, $P$ and $Q$ will be left for third forward trip which is not possible
- if $P$ comes back in second return, it means $Q$ must have come back in first return, which again means both $P$ and $Q$ are left for the final trip.

- Since $Q$ cannot come back, it means both $Q$
**and**$R$**must go together in the forward trip**.

- Suppose $P,S$ goes first and $S$ rows.
- $P$ comes back and then $Q$ and $R$ returns with $Q$ rowing.
- $R$ comes back and takes $P$ with $R$ rowing.
- Other alternative is $Q$ and $R$ going first.
- In both cases $R$ has to row twice.

Correct Option: C.

+8 votes

By going through the above criterion we can conclude the following statements

In the first forward trip, Q and R will go and on this trip, Q will row.

Now, R will come back on the 1st return trip and R will row.

In the 2nd forward trip, R and P will go and R will row.

Now, P will come back on the 2nd return trip and P will row.

In the 3rd forward trip, P and S will go and S will row.

**So, R rowed Twice. option C)**

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