Question 112921
SEE IF YOU CAN FOLLOW MY APPROACH, BARB. I LIKE TO LAY OUT WORD PROBLEMS SO I CAN CLEARLY SEE WHAT THE UNKNOWNS REPRESENT AND HOW THEY RELATE TO OTHER COMPONENTS OF THE PROBLEM.
Let x=number of minutes a person can talk without exceeding $3.
Now we are told that the cost of talking x minutes=$0.36+0.21(x-1) and we are also told that this has to be less than or equal to $3.  So our inequality is:

$0.36+$0.21(x-1)<=$3 get rid of parens

$0.36+$0.21x-$0.21<=$3  simplify 
$0.15+$0.21x<=$3 subtract $0.15 from both sides

$0.15-$0.15+$0.21x<=$3-$0.15  collect like terms

$0.21x<=$2.85  divide both sides by $0.21

x<=13.5714 minutes but we would be charged for 14 minutes

14 minutes would cost $0.36+13*$0.21=$0.36+$2.73=$3.09 and this breaks the bank so we have to round down to the nearest minute:

x<=13 minutes---------ans

CK

$0.36+12*$0.21=$0.36+$2.52=$2.88 and
$2.88<=$3.00------------------one more minute or portion thereof would break the bank


Hope this helps---ptaylor