First you have to find out how much money you have put in this month, so you need to add your two deposits together.
24.61 + 16.49 = $41.10
Now we know that this month you put $41.10 in your account, but there is a total of $72.31 in your account. Now you need to subtract what you put in this month to your total amount.
72.31 - 41.10 = $31.21 was your balance at the beginning of the month.