`a) 53.39 + 47.39 - 53.21 - 47.21`
`= 53.(39 - 21) + 47.(39 - 21)`
`= 53.18 + 47.18`
`= 18.(53 + 47)`
`= 18.100`
`= 1800`
`5 . 7 . 77 - 7 . 60 + 49 . 25 - 15 . 42`
`=5 . 7 . 77 - 7 . 5 . 12+ 49 . 5 . 5 - 3 . 5 . 42`
`=5 . 539 - 84 . 5 + 245 . 5 - 5 .126`
`=5 . (539-84+245-126)`
`=5 . 574`
`=2870`
#dht151