You have bank accout with balance of $$100$

Have two transactions $($Transaction A and Transaction B$)$ ouccur nearly simultaneosuly:

Transaction A: Deposit $$50($balance $+$ $$50)$

Transaction B: Withdraw $$20($balance $-$ $$20)$

Both transactions read the initial balance $($$$100),$ and their oprations are supposed to update the balance accordingly.

How the execution of the transactions results in different values of the balance amount?