how can you know you'll win 10 out of 54 of the time?
Harrington says that if he has a bigger pair, you will still win 10 out of 54. That's about the odds of you making your set to beat his pair, without him making something bigger.
That is, if he has K/K, you'll still win the hand (most likely with a 5 on the board) about 10 out of 54 times.
As for the problem and figuring it out...
Someone limps for $100.
You have $2,100 and call with 5/5.
The SB shoves the rest of his stack in ($550 total).
It's folded back to you.
$850 pot, $450 to call.
First off, Harrington tries to put him on a range of possible hands. In this case he figures Q/J+ if the guy is tight.
Winning those 10 out of 54 overpairs, and 52 of the 96 unpaired hands he could have means that you'll have 62 winners for every 88 losers.
That gives you odds to call.