Q18. Mrs Lim baked some tarts. She packed all the tarts in either small or big boxes. She packed 7 tarts in each small box and 10 tarts in each big box. At first, there were 4 times as many small boxes as the big boxes. After selling half of the number of small boxes of tarts and some big boxes of tarts, Mrs Lim gave her friends another 8 small boxes of tarts. The number of tarts in the unsold small boxes was thrice the number of tarts in the unsold big boxes. There were 280 tarts left in the big and small boxes. How many tarts did Mrs Lim bake at first?

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