The half-life of a radioactive substance is  30 minutes. The time (in minute) taken between 40% decay and 85% decay of the same redioactive substance is

(a) 15           (b) 30

(c) 45           (d) 60

(d) Key idea Half-life of a radioactive substance is T12log NoN

Given,     N1=0.6N0                 40% decayN2=0.15N0               85% decay

Putting these in the formula

        N2N1=0.15N00.6N0=14=142

So, two half-life periods has passed.

Thus, time taken=2×t1/2=2×20=60 min

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