lets look at the amount earned.
WHAT DO I KNOW?
I know that 6% of $x + 3% of whats left ($1600x) = $78
Now REMEMBER.... PERCENTS ARE ALWAYS HUNDRETHS!
So 6/100 times x + 3/100 times (1600x) must = $78
now we have (6/100) * x) + (3/100) *1600)  (3/100) * x = 78
then 6x/100 +4800/100  (3x/100) = 78 or 6x/100  3x/100 +4800/100 = 78
therefore 3x/100 + 48 = 78 or 3x/100 = 30 or 3000 = 3x and therefore x = 1000.
The amount in the mutual fund that earned 6% was 6% of $1000 = $60
The amount in the mutual fund earned 3% was 3% of $600 = $18
totaling $78.
so $1000 earned 6% and $600 earned 3%
There are other ways to do this, but this is pretty straight forward.
8/7/2013

Stuart R.