`a ) 4/7 × 3/5 × 7/4 × 20 × 5/36`
`= {4 × 3 × 7 × 20 × 5}/{7 × 5 × 4 × 36}`
`= {3 × 20}/36`
`= 60/36`
`b ) 1/30 + 1/42 + 1/56 + .... + 1/132`
`= 1/{5 × 6} + 1/{6 × 7} + 1/{7 × 8} + ... + 1/{11 × 12}`
`= 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + ... + 1/11 - 1/12`
`= 1/5 - 1/12`
`= 7/60`
`c ) 2/{4 × 7} + 2/{7 × 10} + 2/{10 × 13} + ... + 2/{73 × 76}`
`= 2/3 × ( 1 - 1/4 + 1/4 - 1/7 + 1/7 - 1/10 + 1/10 - 1/13 + ... + 1/73 - 1/76 )`
`= 2/3 × ( 1 - 1/76 )`
`= 2/3 × 75/76`
`= 150/228`
`#dtkc`
