Here is a table showing some ways of obtaining a £5 increase:

 Deposits $(+£2)$ Withdrawals $(-£3)$ Calculation Outcome ... ... ... ... $4$ $1$ $4 \times (+£2) + 1 \times (-£3)$ $+£5$ $7$ $3$ $7 \times (+£2) + 3 \times (-£3)$ $+£5$ $10$ $5$ $10 \times (+£2) + 5 \times (-£3)$ $+£5$ ... ... ... ...

Can you continue the table in each direction?