Mathematics · Oct / Nov 2025 · Paper 41 · Question 19
0580
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The mass of a radioactive substance decays exponentially at a rate of 10% per day. The initial mass of the substance is 20 g. Find the number of whole days it takes for the mass of the substance to first be less than 1 g. days [3]
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29 [3]
B2 for 28.4[3…] OR
M2 for 20 × 0.928 or 20× 0.929 evaluated to at least 1 dp or for 0.928 or 0.929 evaluated to at least 2 dp
or M1 for at least two trials of [20 ×] 0.9n soi n 10 > − or for 1 20 1 100 oe
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