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If Ali has x cards, then Belinda has 2x cards. 
If Ali has x cards, then Charlie has x+5 cards. 
2x (Belinda) + x+5 (Charlie) + x (Ali) = 33 (total)
2x + x+5 + x = 33
Now simplify the equation... x+x+5 can be simplified as 2x+5. 
2x+5 plus 2x can become 4x+5. 
So their total number is 4x+5. 
So 4x+5=33. 
You want to isolate the variables and numbers. 
To remove the 5 from the left side, subtract 5 from both sides. 
4x+5-5=4x - 33-5=28
So the equation now is 4x=28
To remove the 4, divide each side by 4. 
4x÷4=x - 28÷4=7
So x equals 7, Ali has 7 cards. 
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