An empty box given as W, weighs 250 grams. Also it can be filled with an unknown number of t-shirts, but what we do know is that each t-shirt weighs 132.5 grams. Therefore, the equation to represent the relationship between W (weight of the filled box) and T (the number of t-shirts) is given as
W = 250 + 132.5T
This equation means that for every value of T, that is, for every number of t-shirt) the value of W would vary/change. Two possible solutios therefore would be;
(1) Let T = 1, that is if there is one t-shirt in the box, then you'll have
W = 250 + 132.5 (1)
W = 250 + 132.5
W = 382.5
(2) Let T = 2, that is if there are two t-shirts in the box, then you'll have
W = 250 + 132.5 (2)
W = 250 + 265
W = 515
As you must have noticed, T can take any number of values, it can be 1, or even 1000 or possibly more. So T is a variable (the value can vary). W also increases as the value of T increases. That means W is also a variable, but it only increases as T increases. If you increase your T to 3, the value of W would go higher. So W is a variable that depends on T, and that makes W a "dependent variable."
Therefore, when T = 1, W = 382.5, and T = 2, W = 515, and so on.
The solutions mean that the value of W varies directly with the value of T. Increase T and W would increase, reduce T and W would also reduce.