is my answer right?

Q) Using a frame rate of 25 fps, what frame number will AviSynth give to the final frame of a 6-second video clip?

A. 25*6=150

Sounds right to me, yes.

Right.

Yes, your answer is correct. To arrive at the frame number, you need to multiply the frame rate with the duration of the video clip in seconds. In this case, the frame rate is 25 fps and the video clip is 6 seconds long. By multiplying these values together (25 * 6), you get the final frame number, which is 150.