Khan Academy: https://www.khanacademy.org/math/algebra-basics/core-algebra-systems/core-algebra-systems-word-problems/v/using-a-system-of-equations-to-find-the-price-of-apples-and-oranges
IXL: https://au.ixl.com/math/year-10/solve-simultaneous-equations-using-elimination-word-problems
More: https://www.ixl.com/math/algebra-1/solve-a-system-of-equations-using-any-method-word-problems