Tammy needs to give 400 cards to Qiulian for both of them to have the same number of cards, which would result in each having 1200 cards.
If Qiulian and Tammy have a total of 2400 cards, and Tammy has twice as many cards as Qiulian, let's set up equations to represent this:
Let Q represent the number of cards Qiulian has.
Let T represent the number of cards Tammy has.
From the information given, we have two equations:
Q+T=2400 (The total number of cards they have together is 2400.)
T=2Q (Tammy has twice as many cards as Qiulian.)
Substitute equation 2 into equation 1 to solve for Qiulian's number of cards:
Q+2Q=2400
3Q=2400
Q=800
So, Qiulian has 800 cards. If Tammy has twice as many cards, she has
2×800=1600 cards.
To find how many cards Tammy must give to Qiulian so they have the same number:
For them to have an equal number of cards, the difference between Tammy's and Qiulian's cards needs to be halved. The difference is
1600−800=800 cards. Half of this difference is
800÷2=400 cards.
So, Tammy needs to give 400 cards to Qiulian for both of them to have the same number of cards, which would result in each having 1200 cards.