Cary and her friends Tasha both had 20 tickets each to play games. Cary used x of hers and Tasha used 2 times as many as Cary. When Cary's dad doubled the combined amount they had left, they ended up with 39 tickets. How many tickets (x) did Cary use?

Cara had (20-x) tickets that left.

Tasha had (20-2x) tickets that left.

Tasha and Cara together

(20-x) + (20-2x) = 20+20-x-2x=40-3x

Tasha and Cara together had (40-3x) tickets that left.

Dad doubled it

2 * (40-3x)

Now they have

(40-3x) + 2 * (40-3x) = 39

40-3x+80-6x=39

120-9x=39

9x=120-39

9x=81

x=9

Cary used 9 tickets.