I give 1/8 of my stamps to Tom. If I gave Tom 15 stamps, how many stamps did I have at first?

1/8 of x = 15

1/8x=15

0.125x=15

0.125x/0.125=15/0.125

x = 120

You had 120 stamps to begin with.

1/8 of all of your stamps = 15

Lets call 'all of your stamps' as s

And also remember that 'of' means multiplications so:

1/8 x s = 15

Multiply 8 on both sides:

s = 15 x 8

s = 120

So you have 120 stamps originally.