Ding Ding, Liang Liang and Jun Jun went to the bookstore to buy "Space Wars". If you buy three copies with the money brought by Tintin, there is still a difference of 55 yuan; if you use the money brought by Liangliang to buy three copies, there is still a difference of 69 yuan; It is known that Junjun brought 37 yuan, so how much will it cost to buy a copy of "Space Wars"?
丁丁、亮亮和军军三人去书店买《太空大战》．如果用丁丁带的钱买三本，还差55元；用亮亮带的钱买三本，还差69元；用三人带的钱去买，就多30元．已知军军带了37元，那么买一本《太空大战》要用多少钱？
<gadget id="calculator">30 - 37</gadget>
<output>-7</output>

<gadget id="calculator">55 + 69 + (-7)</gadget>
<output>117</output>

<gadget id="calculator">117 / 3</gadget>
<output>39</output>

<result>39</result>