## modulus, modular multiplicative inverse, RSA protocol

In web article I've been reading, they talk about the modulus of 23 x 31 = 713. Then show the equation:
d = 19^-1mod(22 x 30) = 139.
I want to know how to calculate d. I don't know how multiplication gives a modulus. I thought the modulus was the remainder of a division problem.
Thanks.