Pure water has a ph of 7, but some cases of acid precipitation have been recorded with a ph close to 3. this means that acid rain has a concentration of hydrogen ions that is times the hydrogen ion concentration of pure water.

The answer to this question would be 10000
The formula to find pH is using log which mean that for every 1 less pH it will be 10 times more hydrogen ion concentration. In this case, pH of pure water is 7 and the acid rain is 3. That means the acid rain water has higher hydrogen concentration than pure water. The calculation for the ratio would be:
10^7-3= 10^4