[(38×a+120):25×54]-980=100
[(38×a+120):25×54]=100+980
(38×a+120):25×54=1080
(38×a+120):25=1080:54
(38×a+120):25=20
38×a+120=20×25
38×a+120=500
38×a=500-120
38×a=380
a=380:38
a=10
