A certain radioactive isotope is a by-product of some nuclear reactors. Due to an explosion, a nuclear reactor experiences a massive leak of this radioactive isotope. Fortunately, the isotope has a very short half-life of 7 days. Estimate the percentage of the original amount of the isotope released by the explosion that remains 4 days after the explosion?
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(1/2)^(4/7) =~ 67.295%