Answer:
$1320
Explanation:
Here we are given that from January to December , he made some investments and a withdrawal.
Investments : $12000 + $2400 + $2880
= $17280
Withdrawal = $1440
Total Balance in the investment fund at the year end = $17280-$1440
= $15840
Total Number of Month in a year = 12
Average Monthly income =

=1320
Hence his average monthly investment is $1320