<PRE><B>Question 221017</B>
A disk jockey must play 12 commercial spots during one hour of a radio show.
 Each comercial is either 30 seconds or 60 seconds long. 
If the total commercial time during one hour is 8 minutes, how many 30-sec commercials were played that hour? 
How many 60-sec commercials?
:
Let x = no. of 30 sec
let y = no. of 60 sec
:
Convert 8 min to 480 second
:
Two equations:
x + y = 12
arrange for substitution
x = 12-y
:
30x + 60y = 480
simplify, divide by 30
x + 2y = 16
;
Replace x with (12 - y)
12 - y + 2y = 16
-y + 2y = 16 - 12
y = 4 ea 60 sec commercials
and
x = 12 - 4
x = 8 ea 30 sec commercials
;
Check solution in 2nd original equation
30(8) + 60(4) =
240 + 240 = 480 sec = 8 min</PRE>