FR = 0.5n
FR = fraction remaining = ?
n = number of half lives that have elapsed = 54 days x 1 half life/6.75 days = 8 half lives have elapsed
FR = 0.58
FR = 0.003906 which is the fraction remaining
Converting this to mg, we have 50 mg x 0.003906 = 0.195 mg = 0.2 mg (1 sig. fig. based on 50 mg value)