Duncan checks his investments everyday. One friday, he missed the news and did not know if there was a gain or a loss. His mother told him that the overall value dropped $210 during the week . That was his gain or loss on the friday.
Monday Tuesday Wednesday Thursday Friday
+300 -193 -25 +51 ?
The overall value of the week can be found by summing all investments. Let X be the investment on Friday, then I(overall) = +$300-$193-$25+$51+X = -$210; 133+X = -210; X = -343; Thus on Friday there was loss of $343.