Need to extend content of project to match duration time

i have ae project slide show 54.16 seconds long. want extend 1 minute, 20 seconds long. have gone composition settings , changed duration 1 minute, 20 seconds, changes length not stretch content fill new length. how stretch content fill new extended time? have exhausted google searches , youtube videos , out of options @ point. please provide solution. thank you.

by far easiest thing nest 54 second comp in new comp 1 minute , 20 seconds long  , enable time remapping. put 2 keyframes in timeline. grab last 1 , drag end of timeline plus 1 frame put cti @ end of comp , press alt/option + ] set layers out point cti.  there it.

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