3400 stickers are in 3 packets.
The 1st packet has 400 stickers more than the total number of stickers in the 2nd and 3rd packets.
The 3rd packet has 4 times as many stickers as the 2nd 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 2nd packet = 1 u
Number of stickers in the 3rd packet = 4 u

Number of stickers in the 2nd packet and 3rd packet
= 1 u + 4 u
= 5 u

Number of stickers in the 1st packet = 5 u + 400

Total number of stickers
= 1 u + 4 u + 5 u + 400
= 10 u + 400

10 u + 400 = 3400
10 u = 3400 - 400
10 u = 3000

1 u = 3000 ÷ 10 = 300

Number of stickers in the 1st packet
= 5 u + 400
= 5 x 300 + 400
= 1500 + 400
= 1900

Answer(s): 1900