.
Let x be the Bob' speed, in kilometers per hour.
Then the Sam' speed is (x-0.7) km/h.
Bob' swimming time is hours.
Sam' swimming time is hours.
The difference of swimming times is 15 minutes = of an hour.
It gives THIS "time" equation
- = .
To solve it, multiply both sides by 4x*(x-0.7). You will get
8x - 8*(x-0.7) = x*(x-0.7)
5.6 = x^2 - 0.7x
x^2 - 0.7x - 5.6 = 0
= = = .
Of the two roots, only positive value does work x = = 2.74.
So, the ANSWER is: the Bob' speed is 2.7 km/h; the Sam' speed is 2.7 - 0.7 = 2.0 km/h (both values are rounded).
Solved.
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I hate using distance-speed-time charts.
Teaching this way is the same as teaching small healthy baby to walk using crutches.
Then the student thinks about this table, but not about the problem.
My criterion is: if the student needs/uses such table, it means that he (or she) does not know the method
and does not know the right approach to the problem.
Therefore, I teach via the LOGIC of the problem, hoping that in this way it will go through the student's mind.
It is how my teachers taught me in my secondary school.
They were masters of teaching (!)