Level 15 is difficult and it took me fifteen minutes to understand what really is happening there. I will post screenshot and explain how to open door 15.
Select the stick from the back of the plant on the left extreme.
Use the stick and switch on the ceiling light, you will see a series of small lights popping up.
As shown in the screenshot below, tap the small light to turn green depending on the no. of branches on the cactus(tree). 2, 4, 1, 3.
Tap the second button from the left, then fourth button, then first and finally third.
Door will Unlock, move to the next level.
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