Let Rs. $\mathrm{x}$ be invested in the first bond. Then, the sum of money invested in the second bond pays

Rs  $(30000-x)$.

It is given that the first bond pays $5 \%$ interest per year and the second bond pay $7 \%$ interest per year.

Therefore, in order to obtain an annual total interest of Rs. $1800$ , we have :

$[x(30000-x)]\left[\frac{5}{100}\right]=1800$     $\left [S I \text { for } 1 \text { year }=\frac{\text { Principal } \times \text { Rate }}{100}\right]$

$\Rightarrow $  $\frac{5 x}{100}+\frac{7(30000-x)}{100}=1800$

$\Rightarrow $ $5 x+210000-7 x=180000$

$\Rightarrow $ $210000-2 x=180000$

$\Rightarrow $ $ 2 x=210000-180000$

$\Rightarrow  $ $2 x=30000$

$\Rightarrow $  $x=15000$

Thus, in order to obtain an annual total interest of Rs $1800$ , the trust fund should invest Rs. $15000$

in the first bond and the remaining Rs. $15000$ in the second bond.