Einstein riddle - 1

RED + ORANGE + YELLOW + VIOLET = 13
A + B + C + G = 13
RED + A + GREEN + D = ?
Hint: "Order and simplification are the first steps towards mastery of a subject"
Thomas Mann

RED + ORANGE + YELLOW + VIOLET = 13
A + B + C + G = 13
RED + A + GREEN + D = ?
Hint: "Order and simplification are the first steps towards mastery of a subject"
Thomas Mann