I added a pic of how I would have done it. Hope it helps :)

Thank for the picture but may i know why u cut into 5 small parts per unit

It was a matter of working backwards. Originally, my ratio was 3 u lemon tarts to 2 u of apple tarts.

So, after I knew the end ratio was 6 u lemon tarts to 1 u of apple tarts, I needed to add the same number of red units to both lemon and apple tarts till I get back the original ratio.

:) For me, it was just a matter of coincidence that each original unit was nicely divided into 5 smaller u. I probably wouldn't have deduced this straightaway without drawing out the model bit by bit.

Thank for the clarification , it help a lot

You're welcome :) glad it helped!