let the first number be x
let the second number be y
x+y=203203------eqn1
x-y=3131----------eqn2
subtract eqn2 from eqn1. you'll have
2y=200072
y=100036
substitute y=100036 in eqn1, you'll have
x+(100036)=203203
x=203203-100036
x=103167
∴ x(the first number)=103167
y(the second number)=100036
