What is the value of x?

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A33 gram sample of a substance that's used to detect explosives has a k-value of 0.1473. n = noekt no = initial mass (at time t = 0) n = mass at time t k = a positive constant that depends on the substance itself and on the units used to measure time t = time, in days

Math i have no idea how to do a and b

Charlie tosses five coins.what is the probability that all five coins will land tails up.