Then x-pull is the frequency of frontal appearance, and the mathematical expectation and variance of x-pull are EX=0.5, DX= 1/4n.
p(0.4 & lt; X pulling force
2 φ (0.2 √ n)-1> if the probability of frontal appearance frequency in the range of (0.4, 0.6) is not less than 0.9; =0.9
φ(0.2√n)>=0.95, 0.2√n >= 1.65, so n >; =68.0625, so throw it at least 69 times.