Answer: at least 87.74
Explanation: We can model this with an equation where your balance must be at least 250. "At least" means greater than or equal to, so the account balance must be greater than or equal to 250. The balance is your starting amount minus any purchases and plus any deposits, so:
286.14 - 123.88 + x >= 250
Where x is what you need to deposit
This simplifies to
162.26 + x >= 250
x >= 87.74