Question 144142
{{{C=0.25(t-400) + 44.95}}} Start with the given equation



{{{91.20<=0.25(t-400) + 44.95<=114.70}}} Set the right side greater than or equal to than the lower endpoint, but less than or equal to the higher endpoint




{{{91.20<=0.25t-100 + 44.95<=114.70}}} Distribute



{{{91.20<=0.25t-55.05<=114.70}}} Combine like terms



{{{146.25<=0.25t<=169.75}}} Add 55.05 to all sides



{{{585<=t<=679}}} Divide all sides by 0.25 to isolate t



So if the time is between 585 and 679 minutes, then the cost will be between $91.20 and $114.70.