Most of the required information's are already given in the question.
Cost of the Television bought by the man = 300 pounds
Percentage of deposit that was made = 15%
Then
amount of deposit made by the man = (15/100) * 300 pound
= 15 * 3 pounds
= 45 pounds
So the man made a deposit of 45 pounds. I hope the procedure is simple enough for you to understand. You can always use this procedure for doing similar problems in future and you will require no additional help from anybody.