Respuesta :

n= Heidi's marbles

92 - n =18   (Barry's 92 marbles - whatever Heidi has = 18 marbles difference)

92 -n +n = 18 +n   (I added n to each side)

92 = 18+n    

92 -18 = 18-18 + n   (I subtracted 18 from both sides)

74 = n    Heidi has 74 marbles

Hope this helps!