1)50+а+б=а+(б+50)
2)d-18÷2<d-18:3
3)25+а+б+д=а+д+(б+25)
4)д-6×2=д-2×6
7/13<a<8/13, 7/13=21/39, 8/13=24/39.
21/39<a<24/39, a=22/39, 23/39/