RC Circuit Question! Kinda Confused one Which Equation to Use.?

Question

Question: In a heart pacemaker, a pulse is delivered to the heart 81 times per minute. the capacistor that controls this pulsing rate discharges through a resistance of 1.8e6 ohms. one pulse is delivered every time the fully charged capacitor loses 63.2% of its original charge (CONFUSING!). what is the capacitance of the capacitor?

My Approach:
I went ahead and converted 81 pulses/minute to pulses/second (which is like 1.35 pulses/sec). I am having trouble plugging these numbers into an equation - q = q(initial)*e^(-t/RC). im not sure that's the equation to use. i would appreciate all the help i can get on this problem! even a fully solved problem would be cool.

Answer 1

63.2% is one time constant (1 - e^-T : T=1) (discharge formula)

t = RC
1/1.35 sec = 1.8e6 * C
0.740 = 1.8e6 * C
411 nF = C

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