Question

Iodine-125 has a half-life of about 60 days.

How many milligrams of a 1000mg sample will remain after 240 days?

Answer 1

62.5 mg

Explanation:

Half-life of Iodine-125 is 60 days

It means the sample loses weight twice in every 60 days

Solution 1

This is a GP with first term of 1000 mg, common ratio of 1/2 and number of terms:

• 240 days/ 60 days = 4

Last term is the remaining sample after 240 days:

• 1000*(1/2)^4= 62.5 mg

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Solution 2

• Day 0 = 1000 mg
• Day 60 = 1000 mg/2= 500 mg
• Day 120 = 500 mg/2= 250 mg
• Day 180 = 250 mg/2= 125 mg
• Day 240 = 125 mg/2= 62.5 mg

Answer is: 62.5 mg of Iodine-125 remains after 240 days