What we must first identify is what value is unknown to us in this question. Here, we don't know how many days have passed until B is fully cut. So we will call this value "X". This is tricky if we want to draw a diagram to help us solve this, as we don't know how many "cuts" in our diagram we should make in the first place.

The trick is to realise that cutting 4 meters of wire every X days is equal to cutting X meters of wire every 4 days. You can see this by multiplying 4 meters with X days to find that the length of B is 4X meters, then apply the same logic to A to see that A is 3X + 54 meters long. Now, we use this rearranged information to draw our diagram to help us find our answer easier.

From there, we simply follow our standard diagram method to solve for what X is equal to, and once we know that, we have all the information we need to see the full picture.

Hope this is clear enough to follow, let me know if you have any queries still!