Explanation:
Step one:
given data
we are told that she wants her balance to be at least $100
y=100-------1
the expression relating her current balance and spending rate is
let y be the total and m be the number of months
y=330-11m----------2
Required
the number of months m
equating 1 and 2 we have
100=330-11m
11m=330-100
11m=230
m=230/11
m=20.9
m=21 approx
This means that she will keep her account open for 1 year 9 months