An insect crect crawling up a wall crawls ` 6cm` upwards in the first minute but the slides `4 cm `downwards in the next minute. If again crawls up `6 cm` upwards in the third minute but again slides `4 cm` downwards in the forth minute. How long will the insect take to reach a crevice in the wall at a height of `22 cm` from a starting point ?`