2100 stickers are in 3 packets.
The 3rd packet has 900 stickers more than the total number of stickers in the 2nd and 1st packets.
The 1st packet has 3 times as many stickers as the 2nd packet.
How many stickers does the 3rd packet contain?
Choose the packet with the least stickers to be 1 u.

Number of stickers in the 2nd packet = 1 u
Number of stickers in the 1st packet = 3 u

Number of stickers in the 2nd packet and 1st packet
= 1 u + 3 u
= 4 u

Number of stickers in the 3rd packet = 4 u + 900

Total number of stickers
= 1 u + 3 u + 4 u + 900
= 8 u + 900

8 u + 900 = 2100
8 u = 2100 - 900
8 u = 1200

1 u = 1200 ÷ 8 = 150

Number of stickers in the 3rd packet
= 4 u + 900
= 4 x 150 + 900
= 600 + 900
= 1500

Answer(s): 1500