The first method: 40t 1+4t2=7.
4t 1+4t3+40(t3+t2)=7
40t 1-50t3=4t 1+4t3
The second method: (7-s 1-s2)/50=s3/4.
(S2-S3)/4+S3/4+(7-S2)/40 =(7-s 1)/40+s 1/4
(s2-s3)/4=(7-s 1)/40