She has 648 shirts.
Work:
Lisa has 42 pineapples. 4 + 2 = 6
Johnny has 6 marbles.
42 + 4 + 2 + 6 = 54
54 + 5 + 4 = 63
63 + 6 + 3 = 72
But 72 is a multiple of 6!
Let's go further:
72 * 6 = 432
Now let's look at the word 'shirt'.
The middle letter is I.
I is the 9th letter of the alphabet.
Remember the number 72?
72*9 = 648