diff --git a/docs/mystery/3_fertility_policy.md b/docs/mystery/3_fertility_policy.md
index 097c337..effa3bf 100644
--- a/docs/mystery/3_fertility_policy.md
+++ b/docs/mystery/3_fertility_policy.md
@@ -152,10 +152,13 @@
         $$
             E(X) = \frac{1}{p} = 2
         $$
+
         因为男孩数量为1，所以女孩数量为$Y=X-1$，所以有:
+
         $$
             E(Y) = E(X-1) = E(X) - 1 = 1
         $$
+
         即每个家庭平均有1个女孩，所以男女比例为1:1。