The total area of a series of squares arranged from small to large is 3500 square meters. The side length of the smallest square is 5 meters, the side length of the next square is 10 meters, and the side length of each subsequent square increases gradually by 5 meters. How many squares are there in total?
从小到大排列的一系列正方形面积的总和为3500平方米．最小的正方形边长为5米，下一个正方形的边长为10米，以后每个正方形的边长按5米逐渐增加．问一共有多少个正方形？
<result>7</result>