Multi just means "more than one".
Dual means "two".
So a dual-core CPU has 2 CPU cores, no more, no less.
A multi-core CPU has 2+, you can buy CPU's with 2/3/4/6/8 cores these days I believe so it really depends specifically on which multi-core CPU your comparing to which dual-core CPU.
And if this is a homework question - write it IN YOUR OWN WORDS ;-)