Answer by taking a quote from the following article:

The 5th Dimension is an American popular music vocal group, whose repertoire includes pop, R&B, soul, jazz, light opera and Broadway--the melange was coined as "Champagne Soul." Formed as The Versatiles in late 1965, the group changed its name to the hipper "The 5th Dimension" by 1966. They became well-known during the late 1960s and early 1970s for their popular hits: "Up, Up and Away", "Stoned Soul Picnic", "Medley: Aquarius/Let the Sunshine In (The Flesh Failures)", "Wedding Bell Blues", "Never My Love", "One Less Bell to Answer", "(Last Night)

The members began rehearsing as the Versatiles in late 1965 and auditioned for Marc Gordon, who headed Motown's Los Angeles office. Although Motown rejected the group's demo tape, Gordon agreed to manage them and brought them to the attention of Johnny Rivers, who had just started his own label, Soul City Records. Their first Soul City single, "I'll Be Lovin' You Forever", was a successful single.  In 1965 The Mamas & the Papas' first single, lead member John Phillips' "Go Where You Wanna Go", failed to open the foursome's chart career. At the suggestion of Rivers and their manager Marc Gordon, the 5th Dimension covered the same song virtually note-for-note (except for the last verse's upward modulation), and their early 1967 version climbed into the top 20 on R&B and pop stations and peaked at #16 on the Hot 100, opening the quintet's chart career.  The budding songwriter Jimmy Webb supplied the group with their breakthrough hit, "Up, Up and Away", a mid-1967 #7 hit that won five Grammy Awards. The following year, the group scored major hit singles with Laura Nyro's songs "Stoned Soul Picnic" (U.S. #3) and "Sweet Blindness" (U.S. #13). The group received a gold record for their album Stoned Soul Picnic.  That album included "California Soul", which peaked at #25 in February 1969. Weeks later the group's success broke wide open, with "Aquarius/Let the Sunshine In" from the musical Hair topping the Hot 100 for six straight weeks in April and May, and another Nyro song, "Wedding Bell Blues", doing the same for the first three full weeks in November. Their cover of Neil Sedaka's "Workin' On a Groovy Thing" went to #20 in between. Those four singles kept the group on the Hot 100 for all but four weeks in 1969. By some reckonings, "Aquarius/Let the Sunshine In" was the biggest hit single for 1969.  Later top 20 hits included 1970's "One Less Bell to Answer" (U.S. #2), 1971's "Love's Lines, Angles and Rhymes" (U.S. #19) and "Never My Love" (U.S. #12), 1972's "(Last Night) I Didn't Get to Sleep at All" (U.S. #8) and "If I Could Reach You" (U.S. #10). The group had seven other top 40 hits, the last being 1973's "Living Together, Growing Together" (U.S. #32) from the film Lost Horizon.

What was their last hit they recorded?
"Living Together, Growing Together" (U.S. #32) from the film Lost Horizon.