求翻译：当密苏里州被作为蓄奴州纳入联邦时，他们又同时允许缅因州（Maine）作为自由州加入。等到1850年加利佛尼亚申请加入时，美国已有15个蓄奴州和15个自由州是什么意思？

When Missouri was included in the federal state as when slavery, they also allow Maine (Maine) to join as a free state.加利佛尼亚 apply until 1850, the U.S. has 15 states and 15 slave-holding states free

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When Missouri was included as slavery States when federal, they also allow Maine (Maine) joined as a free State. Until 1850 when El Capitan, Yosemite Valley, California applied to join, United States there have been 15 slavery States and 15 free State

When Missouri State is regarded as to hold the slave state to include in the union, they allow Maine State (Maine) to join as the freedom state at the same time. While waiting for and applying to join in California by 1850, already 15 have held the slave state and 15 free states in U.S.A.