1900 stickers are in 3 packets.
The 1st packet has 100 stickers more than the total number of stickers in the 3rd and 2nd packets.
The 2nd packet has 3 times as many stickers as the 3rd packet.
How many stickers does the 1st packet contain?
Choose the packet with the least stickers to be 1 u.

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

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

Number of stickers in the 1st packet = 4 u + 100

Total number of stickers
= 1 u + 3 u + 4 u + 100
= 8 u + 100

8 u + 100 = 1900
8 u = 1900 - 100
8 u = 1800

1 u = 1800 ÷ 8 = 225

Number of stickers in the 1st packet
= 4 u + 100
= 4 x 225 + 100
= 900 + 100
= 1000

Answer(s): 1000