V.A.L.A. Psychic

Congratulations, you solved the Kakuro puzzle and figured out V.A.L.A.'s next mission! While preparing to leave Greenland, you spotted someone trying to sneak away. You captured a woman who claims to be a psychic hired by V.A.L.A. She offers to switch sides and work for A.A.K.V.O. and even claims she can prove her psychic powers.

She tells you to pick any number and keep it in your head.  Then she takes out a pad of paper and writes down another number on it but doesn't show it to you. Then she tells you to take your number and subtract 7, then multiply by 3, then add 30, then divide by 3, then subtract the original number.  She tells you that her piece of paper has the result of your calculation and that when she shows it to you and it matches, you have to let her go. Is she really psychic or is it just a trick?