Answer:
Let b = the number of baseball cards he started with
1/2 (b+7) =25
Jason started with 43 baseball cards.
Explanation:
Let b = the number of baseball cards to start with
He bought 7 new cards
b+7
Now the dog at 1/2
1/2(b+7)
There are 25 cards left
1/2 (b+7) =25 This is our equation
Multiply each side by 2
2*1/2 (b+7) =25*2
b+7 = 50
Subtract 7 from each side
b+7-7 = 50-7
b = 43
Jason started with 43 baseball cards.