rmb: total money no change

cost
6 rolls = total + $y
5 rolls + $4 = total

so
6 rolls = 5 rolls + $4 + $y
6 rolls -5 rolls = 5 rolls -5 rolls+$4+$y
1 roll = $4+$y

(a)
1 roll costs $4+$y = $(4+y)


(b)
1 roll = $4+$y
5 rolls = $20 + $y x 5 = $20 + $5y
5 rolls = $20 + $5y

5 rolls + $4 = $20 + $5y + $4
5 rolls + $4 = $24 + $5y

since 5 rolls + $4 = total

5 rolls + $4 = $24 + $5y
total = $24 + $5y = $(24 + 5y)

Mrs J has $(24 + 5y)

.
.
.

Thanks!