`52 . (x : 78) = 3880`
`=> x : 78 = 3880 : 52`
`=> x : 78 = 970/13`
`=> x = 970/13 . 78`
`=> x = 5820`
Vậy `x = 5820`
--
`x . 3/4 + 4/5 = 1 7/10`
`=> x . 3/4 + 4/5 = 17/10`
`=> x . 3/4 = 17/10 - 4/5`
`=> x . 3/4 = 9/10`
`=> x = 9/10 : 3/4`
`=> x = 6/5`
Vậy `x = 6/5`