adalah...............

Un=a+(n-1)b
U5=a+(5-1)b=60
>a+4b=60

U10=a+(10-1)b=50
>a+9b=50

eliminasi
a+4b=60
a+9b=50
------------ -
    -5b=10
      b = -2

subtitusi b=-2
ke a+4b=60
>a+4(-2)=60
>a-8=60
>a=60+8
>a=68

U30=a+(n-1)b
U30=68+(30-1)-2
U30=68+29x(-2)
U30=68-48
U30=20