This is an example of a question that can be solved using algebra and simultaneous equations. By letting the mass of an empty crate be x and the mass of apples in a full crate be y, you can form simultaneous equations in terms of x and y. By making the two equations have the same value of y, we can subtract equation 3 from equation 4 to get an equation in terms of x only. Solve for x.

To respond to the comment on the original solution, there is an easier way to solve this problem using algebra without using two variables. On the left, we can use only 1 variable x and still solve this problem. I apologise for not noticing this earlier.

On the right is a model representation of this question. As can be seen from the model, the shaded region is the proportion of the crate filled with apples. The difference in weight of the two crates is equal to two small squares. Using this information, you can deduce that the weight of the empty crate is 3 small squares subtracted from 40, or 1 small square subtracted from 30.

However, I am not very keen on encouraging students to use the model drawing method after P5. Learning and practising basic algebra early can be very helpful for solving such problems more quickly. Model drawing makes the question easier to visualise, but can be too time-consuming. I hope this helps.