In a pack of construction cardboard, the ratio of blue and red cardboard is 2:7. Jingjing uses 1 blue board and 3 red boards every day. On the last day she used 3 red boards and the last 1 blue board, and left 15 red boards. How many sheets were there originally in this pack of construction cardboard?
在一包建筑用纸板中，蓝色和红色纸板数量比为2：7．晶晶每天用1张蓝色板和3张红色板．最后一天她用了3张红色板和最后1张蓝色板，并且余下了15张红色板．这包建筑用纸板原来共有多少张．
<gadget id="calculator">2 + 7</gadget>
<output>9</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">15 / (1/9)</gadget>
<output>135</output>

<result>135</result>