@wcheng2
The formula for parallel resistors is: R = (R1xR2) / (R1+R2). If R1 is 47K and R2 (your plug in resistor) is 100R, then multiply the two. You will get a large value. Next step, add R1 and R2. You will get a much smaller value. Now divide the larger value with the smaller value to get the final value. Pretty easy.
The formula for parallel resistors is: R = (R1xR2) / (R1+R2). If R1 is 47K and R2 (your plug in resistor) is 100R, then multiply the two. You will get a large value. Next step, add R1 and R2. You will get a much smaller value. Now divide the larger value with the smaller value to get the final value. Pretty easy.