`a, (42+52):2=47`
`b, (36+42+57):3=45`
`c,(34+43+52+39):4=42`
`d,(20+35+37+65+73):5=46`
a, \(\left(42+52\right):2\text{=}47\)
\(b.\left(36+42+57\right):3\text{=}45\)
\(c.\left(34+43+52+39\right):4\text{=}42\)
\(d.\left(20+35+37+73+65\right):5\text{=}46\)