Question 180963
Let x=number of birds on tree B (seven birds leave=x-7)
Then 9x=number of birds on tree A 

We are told the following:
10(x-7)=9x+7
10x-70=9x+7 subtract 9x from and add 70 to each side
10x-9x=77
x=77-----------number of birds on tree B initially
9x=9*77=693--------------number of birds on tree A initially

CK
10(77-7)=693+7
700=700

Hope this helps---ptaylor