The radius of the Outer circle (with r₂) is 6 and the radius of the inner circle (with r₁) is 2. When objects are similar, they have a proportional relationship. If we take the outer circle and squeeze by one third, we get a radius of 6 x 1/3 = 2. On the other hand, if we start with the inner circle and stretch it by 3, we get a radius of 2 x 3 = 6.
Thus, to make r₂ similar, we use a scale factor of 1/3. The third choice is best.
The "k" value should have units and should be negative. k = -.07 / seconds Half-Life = ln (.5) / k Half-Life = ln (.5) / -.07 Half-Life = -.693147 / -.07 Half-Life = 9.9 seconds (actually, wkpedia says half-life of Krypton 91 is 8.57 seconds)
elapsed time = half-life * log (bgng amt / end amt) / log (2) elapsed time = 9.9 * log (100 / 10) / 0.30102999566 elapsed time = 9.9 * 1 / 0.30102999566 <span><span>elapsed time = 9.9 * 3.3219280949
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</span><span><span><span>elapsed time = 32.8870881395
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<span>elapsed time = 32.89 seconds (rounded)
