Question

One number is bigger than other number by 3 and the sum of these two number is 43

Answer 1

$x - \hbox{one number}\\ x+3 - \hbox{ the other number}\\\\ x+x+3=43\\ 2x=40\\ \boxed{x=20}\\ \boxed{x+3=23}$

Answer 2

$x-first\ number\\x+3-second\ number\\x+(x+3)=x+x+3=2x+3-the\ sum\ of\ these\ numbers\\43-the\ sum\ of\ these\ numbers\\\\therefore\\\\2x+3=43\ \ \ \ |subtract\ 3\ from\ both\ sides\\2x=40\ \ \ \ \ |divide\ both\ sides\ by\ 2\\x=20-first\ number\\x+3=20+3=23-second\ number\\\\Answer:\boxed{20\ and\ 23}$