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The way we justify 1 unit = 7/8kg is best seen via a model, but let me try my best with words.
We know that when he uses up 1/8kg of tuna filling, he has 2 units of tuna filling + some extra left.
We know that he has 2 units of Curry to begin with, but once he adds 3/4kg more, he has 2 units + some extra.
Since we know he has an equal amount of each filling after adding and subtracting, we must find what is the “some extra” filling left.
To find this, we realise that you must take the 3/4kg added + 1/8kg used up to find one unit.
If it helps, try drawing it out!
Alternatively, you can like us on www.facebook.com/dingletutors and private message me for the picture of the model 🙂
All the best!