`a, -7/15 . 5/8 . (-15/7) - (-3/8)`
`= -7/15 . 5/8 . (-15/7) + 3/8`
`= -7/15 . (-15/7) . 5/8 + 3/8`
`= 1 . 5/8 + 3/8`
`= 5/8 + 3/8`
`= 8/8`
`= 1`
_____________________
`b, 3/7 + 1/2 - 5/14`
`= 6/14 + 7/14 - 5/14`
`= {6+7-5}/14`
`= 8/14`
`= 4/7`
______________________
`c, -66 . (1/2 - 1/3 + 5/33)`
`= -66 . (33/66 - 22/66 + 10/66)`
`= -66 . 7/22`
`= -21`
_____________________
`d, 4/7 : 2 1/3 + 1 : 21/5`
`= 4/7 : 7/3 + 1 : 21/5`
`= 4/7 . 3/7 + 1 . 5/21`
`= 12/49 + 5/21`
`= 36/147 + 35/147`
`= 71/147`
