Repeated addition would be a very inefficient way to multiply numbers, imagine multiplying 1298654825 by 85324154. Much quicker to just use long multiplication using binary.
1100100
0110111
=======
0000000
-1100100
--1100100
---0000000
----1100100
-----1100100
------1100100
==============
1010101111100
For floating point numbers scientific notation is used.
100 is 1 * 10^2 (10 to the power of 2 = 100)
55 is 5.5 * 10^1 (10 to the power of 1 = 10)
To multiply them together multiply the mantissas and add the exponents
= 1 * 5.5 * 10^(2+1)
= 5.5 * 1000
= 5500
The computer does this using the binary equivalents
100 = 1.1001 * 2^6
55 = 1.10111* 2^5
-> 1.1001 * 1.10111 * 2^(6+5)