Bài $6$:
a) `(x-3)/18`$+$`(x-5)/16`$+$`(x)/21`$=3$
⇔`(56x-168)/1008`$+$`(63x-315)/1008`$+$`(48x)/1008`$=$`(3024)/1008`
⇔$56x-168+63x-315+48x=3024$
⇔$167x-483=3024$
⇔$167x =3024+483$
⇔$167x =3507$
⇔$x =3507:167$
⇔$x =21$
Vậy $S=21$
b)`(x-3)/49`$+$`(x-10)/70`$+$`(x-250)/100`$=4$
⇔`(100x-300)/4900`$+$`(70x-700)/4900`$+$`(49x-12250)/4900`=`(19600)/4900`
⇔$100x-300+70x-700+49x-12250=19600$
⇔$219x-13250=19600$
⇔$219x =19600+13250$
⇔$219x =32850$
⇔$x =32850:219$
⇔$x =150$
Vậy $S=150$
