Muscle strength declines as we grow old. Preliminary evidence suggests that part of this loss of muscle strength may be due to a deficiency of chemical X. Investigators plan a randomized trial to administer chemical X or identical placebo for 5 months to elderly subjects and then measure muscle strength. Previous studies report a mean grip strength in elderly persons of 20 kg with a SD of 5 kg. Assuming alpha (2-sided) = 0.05 and beta = 0.10. What is the sample size?