(a) In a charging RC circuit, how many time constants have elapsed when the capacitor has 99.0%...

Question:

(a) In a charging RC circuit, how many time constants have elapsed when the capacitor has 99.0% of its final charge?
(b) How many time constants have elapsed when the capacitor has 99.90% of its final charge?
(c) How many time constants have elapsed when the current has 1.0% of its initial value?