Apparently only posting the image is not allowed. So welcome to my 2nd puzzle here. I hope this one is a little tougher.
The number you're looking for is:
The reasoning is:
0.5 is the sine of 30 degrees
That sine wave and thermometer are in a square, so 0.5 squared is 0.25
I sure hope there is something clever in this. But, the most obvious thing that comes to mind is:
1/2. Because the graph is of Sin x and the "test-tube" is filled upto 30 degrees. So, Sin 30 =1/2.
The sine of 30 degrees is 1/2 and half of the vessel equals 70/2=35 degrees.