please help I dont understand

Answer:
First team collected 20 books, Second team collected 30 books, Third team collected 15
Step-by-step explanation:
Let us find all equation for n (books)
First team : n - 10
Second team : n
Third team : .3(n + n - 10)
Distributive property :
n+n-10 + .3 * (2n-10)= 65
2.6n = 78
n = 30 (Second team)
n - 10 = 20 (First team)
.3(2n-10)=15 (Third team)