AAMC Spoiler - online practice questions from the official guide

[974679]

View attachment 258316
Can someone please be kind enough to explain this with the general webers formula : change in intensity = intensity x constant
- I understand how 10-12 is the JND = 2 JND, and then going from 10-20 is doubled, so must double the JND, but I would have never thought of this by just looking at the question. Does someone have a different way of understanding this problem?

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[Dream of the Endless]

1. Purely using the equation, it is DeltaI/I = Constant. 12/10 =1.2 which is a constant for all future calculations. X/20 =1.2 --> Which is 24. (1/5 of 20 is 4. Just add it)
I don't understand the second half of your question. Are you trying to understand why you just double it for the JND?

2. If that's your real name, consider a change for anonymity.