The venus flytrap closes in 0.3 second and the waterwheel plant closes in 0.2 second what decimal in halfway between 0.2 and 0.3 ?

In order to find the number in between two numbers you need to calculate their mean. This can be done by adding them up then dividing by 2. In this case, 0.2+0.3 = 0.5. You can then divide this by 2 to give 0.25 
Thus the decimal in between 0.2 and 0.3 is 0.25.