document.write( "Question 1145904: This is a question from my son's test. He got (a) correct but (b)wrong. He doesn't understand why. Would appreciate an explanation that can help him understand his mistake.\r
\n" ); document.write( "\n" ); document.write( "A submarine is at −750 feet or 750 feet below sea level. It descended 300 feet, then ascended 550 feet, and then descended 425 feet.
\n" ); document.write( "a. Write and evaluate a numerical expression to find the submarine’s final elevation. His answer is -925 feet which is correct. \r
\n" ); document.write( "\n" ); document.write( "b. The submarine next descended to the ocean floor, which was at an elevation of −2,250 feet. How far did the submarine descend?
\n" ); document.write( "He answered -1325 feet because according to his understanding the submarine descended so should be negative. However, the correct answer should be 1325 ft (not - 1325 ft). Please help him understand why.\r
\n" ); document.write( "\n" ); document.write( "Greatly appreciate your help, as always. \n" ); document.write( "

Algebra.Com's Answer #767181 by ikleyn(52792) $\"\"$ $\"About$

You can put this solution on YOUR website!
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document.write( "(a)  This sentence and this sequence of maneuvering mathematically is translated into this formula for the elevation\r\n" );
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document.write( "         elevation = -750 - 300 + 550 - 425 = my calculation = -925,\r\n" );
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document.write( "     exactly as your son got it.\r\n" );
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document.write( "     Initial elevation of -750 ft below the sea level is given.\r\n" );
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document.write( "     Descending 300 ft means subtracting 300 ft from the current elevation to get new elevation.\r\n" );
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document.write( "     Ascending 550 ft means adding 550 ft to the current elevation to get new elevation.\r\n" );
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document.write( "     So far everything is very natural.\r\n" );
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document.write( "(b)  The submarine descended from the level of -925 ft to the ocean floor at the elevation -2250 ft.\r\n" );
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document.write( "     How far it descended ?\r\n" );
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document.write( "     Intuitively, it is clear: it descended 2250 - 925 = 1325 ft (use your common sense).\r\n" );
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document.write( "     If you (or your son, or the teacher) wants use mathematic expression (equation) at this point,\r\n" );
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document.write( "      he can write  -925 - descending = -2250,  which gives\r\n" );
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document.write( "                     -925 + 2250 = descending,   or\r\n" );
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document.write( "                     descending = -925 + 2250 =  1325 ft  (the correct answer).\r\n" );
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document.write( "Memorize it:  descending X ft means  new_level = old_level - X  feet;\r\n" );
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document.write( "              ascending  X ft means  new_level = old_level + X  feet.\r\n" );
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\n" ); document.write( "\n" ); document.write( "Hope it helps.\r
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