# The half-life of a radioactive substance is 30 minutes. The time (in minutes) taken between 40% decay and 85% decay of the same radioactive substance is Option 1) 15 Option 2)   30 Option 3) 45 Option 4) 60

As discussed

Number of nuclei after disintegration -

or

- wherein

Number of nucleor activity at a time is exponentional function

Let's say it decays 40% after time t1 and 85% decay after time t2 . Then at t=t1 , N1 = 0.6 N(N0 = initial number of nuclei)

at t = t2, N2 = 0.15 N0

-1

-2

Divide 2 into 1

or

= 60 minutes

Option 1)

15

This option is incorrect

Option 2)

30

This option is incorrect

Option 3)

45

This option is incorrect

Option 4)

60

This option is correct

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