$720=30hrs
$1 = x
cross multiply
$720x=30hr
convert the hour to sec
30*60=1800min
therefore, to sec,=
1800*60=108000s
she takes 720ussd for 108000s
for 1 ussd=108000/720
she takes $1 for every 150 sec
for 40 hrs= 40*3600
=144000
$1 = 150
$x = 144000
x=960
she collects 960 ussd in 40 hours
