Answer :
The best thing to do here is to subtract the difference, which is 4. 42-4=38. You've then got to split them in half, 38/2=19. Therefore, Rob had 19 and Carly had 23
Hope this helps :)
so r+c=42
c=4+r
subsitute c=4+r for r in r+c=42
r+4+r=42
add like terms
2r+4=42
subtract 4 from both sides
2r=38
divide both sides by 2
r=19
rob=19
subsitute r=19 for r in c=4+r
c=19+4
carly=23