Answer
The equation for this question is
84 + 10x = 219
The number of minutes it'll take the submarine to reach 219 feet is 13.5 minutes
Explanation
Let the number of minutes it will take the submarine to reach 219 feet below the surface be x minutes.
The number of feet the submarine reaches in x minutes = (10x) feet
Since the submarine started from 84 feet, in x minutes, it would have reached a depth of
(84 + 10x) feet
This is equal to 219 feet
84 + 10x = 219
Subtract 84 from both sides
84 + 10x - 84 = 219 - 84
10x = 135
Divide both sides by 10
(10x/10) = (135/10)
x = 13.5 minutes
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