The trout is swimming 30 ft below sea level,if it were above sea level it would be positive but since its below it is negative. It then swims 20 feet lower so it would be -50 as a short cut you can use in problems like this one is to ignore the sign's and add it regularly then add the sign after the equation is done.
To get the percentage of area we shall proceed as follows: linear scale factor =length of herb/ length of garden =4/10 but area scale factor=(linear scale factor)^2 area scale factor=(2/5)^2=4/25 thus the percentage area proportion will be: 4/25*100 =16%
