a disk jockey must play 14 commercial spots during one hour of radio show, each commercial is eiter 30 seconds or 60 seconds long. If the total commercial time during one hour is 11 minutes, then ow many 30 second commercials were played that hour?

Answer 1

x of 30 sec so (14-x) are 60 sec
11*60 = 660 sec

so
30 x + 60(14-x) = 660

870 - 30 x = 660

30 x = 210

x = 7

Answer 2

short commercial --- x
long commercial ---- y

x+y = 14 , #1
30x + 60y =11(60)
x + 2y = 22 , #2

#2 - #1
y = 8
then x = 6

6 shorts , 8 long

check : 6+8 = 14
30(6) + 60(8) = 180+480 = 660 = 11(60)

Answer 3

x of 30 sec so (14-x) are 60 sec
11*60 = 660 sec

so
30 x + 60(14-x) = 660

840 - 30 x = 660

30 x = 180

x = 6