7:3时,设原本该数为7r及3r,分子与分母各加70后,成为7:4,所以依题意:7r+70=7k3r+70=4k-->(7/4)(3r+40)=4k (7/4)(21r)/4+490/4=7k7r+70= (21r)/4+490/4,化简合并后得:7r=210r=30所以原数 7x30:3x30=210:90,各加70后 21...