d = r * t

For A :
dA = rA * tA = 7 * 3 = 21 km

For B :
dB = 3 * dA = 3 * 21 = 63 km
Since, B is faster than A by 2 km/hr
rB = rA + 2 = 7 + 2 = 9 km/hr
tB = 63 / 9 = 7 hrs

tB - tA = 7 - 3 = 4 hrs

Hence, A.
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A's rate: \(7\) km/hour
A's time (given): 3 hours
A's distance, \(D=rt=(\frac{7km}{hr}*3hrs)=21\) kms

B's rate: B is faster than A by 2 km per hour
B's rate: \((7+2)=9\) km/hr
B's distance is three times longer than A's
B's distance: \((21km*3)=63\) kilometers
B's time, \(t=\frac{D}{r}=\frac{63km}{9kmh}=7\) hours

Ferry B (7 hours) takes how many more hours than Ferry A (3 hours)?

\((7 - 3) = 4\) hours

Answer A
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In the depths of winter, I finally learned
that within me there lay an invincible summer.
-- Albert Camus, "Return to Tipasa"

The source is my insertion (per the question's tag).
In other words, however good this question may be, it is not an official question.

sameeruce08 , good catch. You wrote
B's distance is 3 times longer than A's
Doesn't it mean that B's total distance is 4 times that of A's distance?

The phrases "3 times longer than" and "3 times as long as" are hotly debated.*

In this case and similar others, translate "times longer than" with the emphases on times -- on multiplication.
Treat "3 times longer than" as a multiplicative comparison, not as an additive or linear comparison.
B's distance = 3 * A's distance
Why?

1) STANDARD PARLANCE

As KarishmaB notes , native English speakers in standard conversation use "3 times longer than" and "3 times as long as" interchangeably.
The two phrases mean the same thing: whatever A is, multiply it by 3. Here: (my emphasis)


2) STANDARD PARLANCE and INTENDED TRANSLATION

Bunuel concurs here: (my emphasis)


3) INTENDED TRANSLATION: ANSWERS give NO OTHER CHOICE

If B were (4 * A), then B's distance would be (4 * 21) = 84 kilometers
B's rate is 9 km/hr. B's time would be 84/9 = 12 hours
A's time is 3 hours.
Time difference between A and B would be (12 - 3) = 9 hours
No answer is "9 hours"

In other words, interpret "B is 3 times longer than A" as
B is (3 * A)
B = 3A or
B is longer than A by a factor of 3

Maybe one example will make "standard parlance" more understandable:

5 days is 5 times longer than one day.
In one day there are 24 hours. In five days there are 120 hours. 120 = (5 * 24)
The word "times" refers to multiplication.
At the same time, 5 days is also 5 times as long as one day.

I do not recall having seen an official question that presents this issue, but my recall certainly is not definitive.

Hope that information helps.

*The discussion of X is 3 times longer than Y and X is 3 times as long as Y typically follows this course:
• Interpretation #1: longer by a FACTOR
If X is 3 times longer than Y, then X is longer than Y by a factor of 3. The comparison is multiplicative
Y = 10 miles
X = (10 * [factor of] 3) = 30 miles
• Interpretation #2: longer by a DIFFERENCE
If X is 3 times longer than Y, then X is longer than Y by a difference of 3 times the length of Y. The comparison is additive.
Y = 10 miles
X = 10 + (30) = 40 miles
_________________

In the depths of winter, I finally learned
that within me there lay an invincible summer.
-- Albert Camus, "Return to Tipasa"