Solution: suppose there are m people planting trees on the fourth day, and each race has n trees, then mn trees will be planted on the fourth day.

So on the third day, (m-5) individuals planted trees, and each race planted (n+5) trees, so on the third day, * * * planted (m-5)(n+5) trees.

Similarly, the next day, * * * planted (m- 10)(n+ 10) trees;

1 day tree planting (m-15) (n+15);

* * * Planting (m+5)(n-5) trees on the fifth day;

On the 6th day, * * * planted (m+ 10)(n- 10) trees;

On the 7th day, * * * planted (m+ 15)(n- 15) trees.

After planting 3332 trees for 7 days, it is known that: (m-15) (n+15)+(m-10) (n+10)+(m-5)+Mn.

Simplified to 7mn-700=3332, that is, mn=576.

Because 576=32×82 and trees are planted every day, m > 15, n > 15.

So m = n = 24.

So the number of trees planted on the fourth day is 24× 24 = 576.

The number of plants planted in other days is (24-a) (24+a) = 242-A2 = 576-A2 < 576, ①

(where a=5 or 10 or 15).

To sum up: 576 trees were planted on the day with the most trees planted.