<image>A high school in our district conducted a sample survey on the "Love Donation" activity advocated by the student union. The donation situation of the surveyed students is shown in the figure.

(1) A total of ______ students were surveyed in the school.
(2) The frequency of students donating more than 15 yuan is ______.
(3) If the school has a total of 1800 students, estimate the total amount of donations made by all students in the school to be at least how much?

