Assume the third number

Let the third number be $100$.

$N_3 = 100$

Calculate the first and second numbers

The first number is $20\%$ more than $100$, so $N_1 = 100 + 20 = 120$. The second number is $25\%$ more than $100$, so $N_2 = 100 + 25 = 125$.

$N_1 = 120, N_2 = 125$

Calculate the required percentage

We need to find what percentage of the second number is the first number: $\frac{N_1}{N_2} \times 100 = \frac{120}{125} \times 100$.

$\frac{120}{125} \times 100 = 0.96 \times 100 = 96\%$