The first thing we must do for this case is to define variables:
x: the total mass of the chemical in the container
y: a sample of a chemical from a container
We have the following equation:
y = (3/10) x - 5 3/4
Then, for y = 39.1 we have:
39.1 = (3/10) x - 5 3/4
Clearing x:
(3/10) x = 39.1 + 5 3/4
(3/10) x = 44.85
x = (10/3) * (44.85)
x = 149.5 grams
Answer:
the total mass in grams of the chemical in the container before the scientist removed the sample of 39.1 grams was:
x = 149.5 grams