+77 Derrick, John and Kenneth had a total of 720 stickers. Derrick used 20% of his stickers, John used 80 stickers and Kenneth used twice as many stickers as Derrick. They had 448 stickers left. How many stickers did Derrick have at first?
+128656 D + J + K = 720 ⇒ J + K = 720 - D (1)
Derrick used .20D and had .80D left
John had J - 80 left
Kenneth had K - .40D left
.8D + J - 80 + K - .4D = 448 simplify
.4D + J + K = 448 + 80 ,...... sub in (1)
.4D + (720 -D) = 528
-.6D + = 528 - 720
-.6D = -192
D = -192 /( -.6D) = 320 = number Derrick had at first
