As is frequently the case, the problem is not well worded. It should say "solve this problem without any rounding.
As always, translate the problem into mathematical language.
[MATH]\dfrac{x}{100} \cdot y = 400, \text { where } x,\ y \in \mathbb Z, \ 9 < x < 100, \text { and } 99 < y < 1000.[/MATH]
This is a problem of exclusion. Much of the exclusion can be done by common sense, but it can also be done by mathematical formalism. For example, we can exclude from possible y values any value that does not exceed 400. If that is not clear to your unassisted intuition, you can find it mathematically as follows.
[MATH]x = 99 \implies \dfrac{99}{100} < 1 \implies 400 = \dfrac{x}{100} \cdot y < 1 \cdot y = y \implies 400 < y < 1000.[/MATH]
You have just eliminated 301 possible values of y without any trial and error.
Using similar logic you can deduce [MATH]40 < x < 99.[/MATH]
Finally, you can apply an even more powerful mathematical tool, namely prime factorization.
[MATH]y \in \mathbb Z \text { and } {x}{100} \cdot y = 400 \implies y = \dfrac{40000}{x} \implies x \text { evenly divides } 40000 = 2^6 \cdot 5^4.[/MATH]
The only numbers that divide 40000 evenly must themselves be divisible only by some combination of powers of 2 and 5.
[MATH]40 < x < 100,\ 2 \ | \ x, \text { and } 5 \ | \ x \implies 50 \text { or } 80.[/MATH]
Obviously x = 50 works. How about x = 80?
[MATH]\dfrac{80}{100} \cdot y = 400 \implies 8y = 4000 \implies y = 500.[/MATH]
Only 2 trial and error efforts are needed.