a=2.8b=6.3 b=a/2.8=6.3/2.8=9/4=2.25 a+b=6.3+2.25=8.55
a=2.8b=6.3 a+b=？a=2.8b，2.8b=6.3，说明这三个数都是“6.3”。a=6.3，b=6.3÷2.8=2.25a=6.3，b=2.25a+b=6.3+2.25=8.55a+b=8.55
如图
a+b=2.8+6.3=9.1
