Question

HELP ASAP

Jane took 20 min to drive her boat upstream to​ water-ski at her favorite spot. Coming back later in the​ day, at the same boat​ speed, took her 10 min. If the current in that part of the river is 7 km per​ hr, what was her boat speed in still​ water?

Answer 1

• The speed of the boat in still water was 21 km/h.

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Given:

• The distance is d,
• The speed of the boat in still water is s,
• The speed of the current is 7 km/h,
• Time upstream is 20 min or 20/60 hour,
• Time downstream is 10 min or 10/60 hour.

Set equations for time:

• d / (s + 7) = 10/60 ⇒ d / (s + 7) = 1/6 ⇒ 6d = s + 7,
• d / (s - 7) = 20/60 ⇒ d / (s - 7) = 2/6 ⇒ 6d = 2(s - 7).

Eliminate d and solve for s:

• s + 7 = 2(s - 7) ⇒ s + 7 = 2s - 14 ⇒ 2s - s = 7 + 14 ⇒ s = 21