Answer
$47.4$ grams
Work Step by Step
The half life of DDT is $15$ years.
The number of time intervals in $51$ years is $x=\frac{51}{15}=3.4$.
In each time interval, the initial amount of $500$ grams of DDT decreases by 50%, or is multiplied by $0.5$. There are 3.4 time intervals. Thus:
$y=500(0.5)^{3.4}\approx 47.4$.
