Let the total number of installments be x.

The businessman has refunded 60% of his loan after paying 18 installments. This means that 18 installments account for 60% of the total loan amount.

Each installment represents: $\frac{60}{18} = \frac{10}{3} = 3.33$% of the total loan amount.

Now, to calculate the number of installments for the entire loan (100%): $\frac{100}{\frac{10}{3}} = \frac{100 \cdot 3}{10} = 30$ installments.

Thus, the total number of installments in the agreement is 30.