Let's keep it a problem with one variable
Since Justin has 7.50$ more than Eva, and Emma has 12$ less than justin then we will have:
Eva has X dollars, and justin must have X+7.50$ then Emma should have X+7.50$ -12$ =X - 4.50$
The total is 63$ therefore (Eva's X + Justin's X + 7.50$ + Emma's X - 4.50$) = 63
X + X + 7.50 + X - 4.50 = 63 therefore 3X + 3 = 63 let's divide everything by three to simplify 3X/3 + 3/3 = 63/3
X + 1 = 21 therefore X = 21 - 1 = 20 Eva has 20$, Justin has 20$ + 7.50$= 27.50$ and Emma has 27.50$ - 12$ = 15.50$