Your gas bill went up from $125 per month to $150. what is the percent increase

Take care to read through all the important information's that are given in the question. Based on those information's the answer to the question can be easily deduced.
Initial price of gas = $125
Increased price of gas = $150
Amount of increase in the gas bill per month = (150 - 125) dollars
                                                                        = 25 dollars
Percentage increase in the gas bill per month = (25/125) * 100
                                                                           = 100/5 percent
                                                                           = 20 percent
So the percentage of increase in the gas bill per month is 20%. I hope the procedure is clear enough for you to understand.

The answer is option c.