The distance a spring will stretch varies directly with how much weight is attached to the spring. if a spring stretches 9 inches with 100 pounds attached, how far will it stretch with 90 pounds attached? round to the nearest tenth of an inch

This is the concept of proportionality. To solve the above question we proceed as follows;
let distance be d, weight be w. Thus;
dαw
hence;
h=kw
where;
k is constant;
if the string stretches 9 inches when weight of 100 pounds is places the the formula will be:
9=100k
thus;
k=9/100=0.09
hence;
d=0.09w
The length of the string when 90 pounds is placed will be:
d=0.09(90)
d=8.1 inches