A 100-V source is in series with a 1-M Ω R and a 4-μ F C. a) How much time is requried for Vc to be 63V?

T= RxC 1-M Ω R X 4- μ F C = 4s. This part I get.

b)How much is Vc after 4s? The anwser is supposed to be 86V. But how do I get it? What formula do I use? I have the course book but it doesn't explain how it got 86.

I would appreciate if somone would explain- in detail- how to solve these problems. and/or summarize how you solve for voltage in R/C and R/L.

Thanks