need help

Janet spent 30% of her money on 7 cupcakes and 4 cookies on Monday. The cost of each cupcake was twice the cost of each cookies. She bought some more cupcakes and another 10 cookies with 6/7 of her remaining money on Tuesday. How many cupcakes did she buy on Tuesday?

Let  the  amount  she  started  with  =  A

Let the  cost of a  cookie  =  C    and  the  cost of  a cupcake   =  2C

So

.30A  =  4C  +  7(2C)  ⇒   .30A  =  18C   ⇒   A  =  (18C)  / .30   =   60C   

So   she  had    .70A  left  after  spending  .30A   and   6/7  of this  =  (6/7) .70A =   .60A  = .60(60) C  =  36C

So  with  this   the   amount  she spent  on  cupcakes  on Tuesday  =

36C  -  10C    =     26C  =   13 (2C)

So...she  bought   13  cupcakes