\left \{ {{A+B=70} \atop {A+4=B-3}} \right.
\left \{ {{A=70-B} \atop {70-B+4=B-3}} \right.
\left \{ {{A=70-B} \atop {74-B=B-3}} \right.
\left \{ {{A=70-B} \atop {-B-B=-3-74}} \right.
\left \{ {{A=70-B} \atop {-2B=-77}} \right.
\left \{ {{A=70-B} \atop {B=38,5}} \right.
\left \{ {{A=70-38,5} \atop {B=38,5}} \right.
\left \{ {{A=31,5} \atop {B=38,5}} \right.
…
\left \{ {{6b+s=4,80} \atop {6s+b=11,30}} \right.
\left \{ {{s=4,80-6b} \atop {6(4,80-6b)+b=11,30}} \right.
\left \{ {{s=4,80-6b} \atop {28,80-36b+b=11,30}} \right.
\left \{ {{s=4,80-6b} \atop {28,80-35b=11,30}} \right.
\left \{ {{s=4,80-6b} \atop {-35b=11,30-28,80}} \right.
\left \{ {{s=4,80-6b} \atop {-35b=-17,50}} \right.
\left \{ {{s=4,80-6b} \atop {b=0,50}} \right.
\left \{ {{s=4,80-6*0,5} \atop {b=0,50}} \right.
\left \{ {{s=4,80-3} \atop {b=0,50}} \right.
\left \{ {{s=1,80} \atop {b=0,50}} \right.
…
\left \{ {{L+2T=700} \atop {2L+T=620}} \right.
\left \{ {{L=700-2T} \atop {2(700-2T)+T=620}} \right.
\left \{ {{L=700-2T} \atop {1400-4T+T=620}} \right.
\left \{ {{L=700-2T} \atop {1400-3T=620}} \right.
\left \{ {{L=700-2T} \atop {-3T=620-1400}} \right.
\left \{ {{L=700-2T} \atop {-3T=-780}} \right.
\left \{ {{L=700-2T} \atop {T=260}} \right.
\left \{ {{L=700-2*260} \atop {T=260}} \right.
\left \{ {{L=700-520} \atop {T=260}} \right.
\left \{ {{L=180} \atop {T=260}} \right.