1.

A radioactive atom decays 10% of radioactivity in every 40 minutes. If the current radioactivity of the atom is 35 curie, find what will be its reactivity after 2 hours?1. 22.9 curie2. 23.9 curie3. 19.9 curie4. 20.9 curie

Answer» Correct Answer - Option 1 : 22.9 curie

Given:

Radioactivity of the atom = 35 curie

Decay rate = 10% of present reactivity

Concept:

Since, the candle loses its length after 40 minutes seconds, so it will lose its reactivity 3 times in 2 hours. Use successive decrease percentage formula to calculate the reactivity after 2 hours.

Calculation:

% decrease in reactivity after 40 minutes;

= -10 -10 + (100/100)

= -19%

% decrease after 80 seconds;

= - 19 – 10 + (19 × 10)/100

= -29 + 1.9

= 27.1%

% decrease after 120 seconds;

= - 27.1 – 10 + (27.1 × 10)/100

= - 37.1 + 2.71

= 34.39%

∴ % decrease in radioactivity after 120 minutes = 34.39% 

Radioactivity after 120 minutes = 100% - 34.39% = 65.61% of 35 curie

= 22.9 curie


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