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A study is done to determine if raloxifen will decrease the incidence of vertebral fractures in postmenoposal women. The results are shown:

......................................................Raloxifen..........Placebo......
Number of women in study group.........2557................2576........
Number of vertebral fracture............138(5.4%).......280(10.1%)..

Based on these results, how many women need to be treated to prevent one vertebral fracture?

A) 100/5.4=18.5
B) 100/10.1=10
C) 100/(10.1+5.4)=6.5
D) 100/(10.1-5.4)=21
E) 100/(10.1x5.4)=2

how many women need to be treated=1/absolute risk reduction

absolute risk reduction=event rate(control)-event rate(treated)

event here=no of vertebral fracture/total women in the group

now we substitute the values
event rate for control=10.1%

event rate for treated=5.4%

100/(10.1-5.4)=21

Thanks for the clear explanation, amc.
I also thought the ans is D. But E was the ans on the downloaded form.
Now I'm confident with D.