lets look at the amount earned.

WHAT DO I KNOW?

I know that 6% of $x + 3% of whats left ($1600-x) = $78

Now REMEMBER.... PERCENTS ARE ALWAYS HUNDRETHS!

So 6/100 times x + 3/100 times (1600-x) 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.