Question 959251
w=walked; d=did not walk
w+d=31 (31 days in March)
d=31-w
$1w-$1d=$9 Substitute for d
$1w-$1(31-w)=$9
$1w-$31+$1w=$9 Add $31 to each side.
$2w=$40 Divide each side by $2
w=20 ANSWER She walked the dog 20 days.
CHECK:
d=31-w=31=20=11 She did not walk the dog 11 days.
$1w-$1d=$9
$1(20)-$1(11)=$9
$20-$11=$9
$9=$9