method 1

total = u
1st spending = $6
saved = u-6


2nd spending = 110% of $6 = $6.60
saved = 80% of (u-6) = 0.8u - 4.8
total is still u

6.60 + 0.8u - 4.8 = u
u = $9

ur ans is correct!


OR

method 2

difference in spending
= 6.60 - 6
= $0.60

difference in saving =$0.60
is the difference in % saving

=> 20% = 0.60
100% = $3

total = 6+3 = $9



OR

method 3

spend $6 => saved u
spend $6.60 => saved 80% of u

option (1)
if total is $7,
spend $6 => saved $1
spend $6.60 => saved $0.40
but 0.40 is not 80% of $1

option (2)
if total is $9,
spend $6 => saved $3
spend $6.60 => saved $2.40
80% x $3 is equal to $2.40
option (2) is the right ans!

the following is optional to do

option (3)
if total is $10
spend $6 => saved $4
spend $6.60 => saved $3.40
but 3.40 is not 80% of $4


option (4)
if total is $11
spend $6 => saved $5
spend $6.60 => saved $4.40
but 4.40 is not 80% of $5


...