Total amount = x
Total annual income = 4x/100

Initial amount = 20000
Income generated = 6/100 * 20000 = 1200

Extra amount to invest = y
Income generated = 3y/100

y + 20000 = x ------- equation 1
y = x - 20000 ---- y expressed in terms of x

$1200 + \frac{3y}{100} = \frac{4x}{100}$ ----- equation 2
$1200 + \frac{3(x-20000)}{100 }= \frac{4x}{100}$------ substitute the value of y
$\frac{(120000 + 3x - 60000)}{100}= \frac{4x}{100}$
x = 60000

the additional amount to invest = 60000 - 20000 = 40000

Answer is C.