Please help on this physics question !

strontium-90 has a half-life of 28years. The source contains 100mg of strontium-90.

how long will it be before 75mg of strontium-90 has decayed?

The anwser is 56 years but i do not understand how they got that, help ?!

In 28 years, half will be gone. Down to 50g

In another 28 years (total of 56) another half will be gone. Down to 25g. Total of 75g decayed.

Hope that helps!

Half life is 28 yrs. So after 28 yrs 100 gm will become 50 gm. Again, this 50 gm will become half of it (i.e. 25 gm) in another 28 yrs. So after 28+28 = 56 yrs only 100-50-25 = 25 gm will left. So 75 gms of Stronium-90 will decayed in 56 yrs. There is mathematical formula to calculate it but this will probably easy for you to understand. Take care.

I found a site with the answer and how they got it.. toward the bottom of the page is a chart with your answer for the 56 years at 75mg

http://www.ausetute.com.au/halflife.html

Start with 100mg after 28 years have 50mg, then after another 28 years have 25mg = answer.

You americans dont half get taught some useless crap nowounder use are all brain dead