RC Circuit Question

1. The problem statement, all variables and given/known data
Switch S has been open for a long time. It is then suddenly closed. Determine the time constant (a) before the switch is closed and (b) after the switch is closed. (c) Let the switch be closed at t=0. Determine the current in the switch as a function of time.

2. Relevant equations
T=RC
I=I(e^-t/rc)

3. The attempt at a solution
I actually already know the answers to this question because of my handy dandy answer key. I know part A is the simple T=RC where R = 50kΩ + 100kΩ and C = 10microF. However, I am confused on part B. Why does the resistance for the time constant change from 150kΩ to only using the 100kΩ resistor?

Attached Images
File Type: png Screen Shot 2014-03-18 at 10.14.31 PM.png (67.5 KB)

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