P{至少有1个大于a的概率}＝1－P{所有数都小于等于a}=1-P{X1≤a,X2≤a,X3≤a,X4≤a}
=1-P{X1≤a}P{X2≤a}P{X3≤a}P{X4≤a}=1-a^4=0.9
所以,a=4√ 0.1