All but one of the following sentences demonstrate incorrect style for suspensive hyphenation; revise the style of the number as necessary:
1. Read these tips for space and time-efficient gardening.
2. The issue touches on the tension between middle- and upper-class values.
3. You’re either over- or underwhelmed.
4. This ferry is for Norway-and-Sweden-bound passengers.
5. The marketing campaign targets the 18-to-34-year-old demographic.
When the last word in each of two or more hyphenated phrasal adjectives is the same, all instances but the last may be omitted when it can be clearly understood to apply to all, but retain the hyphen after the words preceding the elision, followed by a letter space.
Original: Read these tips for space and time-efficient gardening.
Correct : Read these tips for space- and time-efficient gardening.
The tips pertain to gardening that is efficient in both space and time, so space should be hyphenated to indicate that the instance of efficient that would normally follow it has been elided.
Original: The issue touches on the tension between middle- and upper-class values.
Correct : The issue touches on the tension between middle- and upper-class values.
Elision of the first use of class is correct.
Original: You’re either over- or underwhelmed.
Correct : You’re either overwhelmed or underwhelmed.
When two words have the same root, spell both words out. (Some guides allow such constructions, such as shown in “He’s scheduled both pre- and postgame briefings,” but it’s best to spell out both words in such a phrase.)
Original: This ferry is for Norway-and-Sweden-bound passengers.
Correct : This ferry is for Norway- and Sweden-bound passengers.
Even if the statement refers to people bound for both Norway and Sweden, rather than one or the other, those passengers cannot visit both countries simultaneously, so they, along with those bound for one country or the other, are referenced in this revision. (“This ferry is for Norway- or Sweden-bound passengers” implies that the destination could be either Norway or Sweden, a statement that would mean the precise destination is unknown.)
Original: The marketing campaign targets the 18-to-34-year-old demographic.
Correct : The marketing campaign targets the 18- to 34-year-old demographic.
The basic rule applies for number ranges. (However, the sentence could be revised to “The marketing campaign targets 18- to 34-year-olds” or “The marketing campaign targets people ages 18–34.”)