Three pawns each and White is winning. The pawn on a5 has no black pawn in front of it, none on the b-file either, and Black’s king is too far away to arrive in time.
7 
2 
g The test takes two seconds
Put a finger on the pawn and look at three files: its own, and the one on either side. If there is no enemy pawn on any of those three files anywhere ahead of it, the pawn is passed. That is the whole definition. The a5 pawn passes the test because Black has nothing on a or b at all; the pawn on f2 fails it, because Black’s f7 pawn is sitting right there.
Beginners forget the neighbouring files. A pawn on d5 with a black pawn on e6 is not passed: that pawn can step to e5 and shoulder it, or simply capture on the way.
Counting the race
Before pushing, count. The pawn on a5 needs three moves to reach a8. Black’s king on g8 needs five to get to a8 or the queening path, and the arithmetic never changes. There is a quicker way to do the same sum by eye, drawing a square from the pawn to the promotion rank, which has its own page on the rule of the square.
The kingside pawns in the diagram are three against three, so nothing happens there. That is the ordinary shape of a won endgame: an extra pawn on one wing, a locked balance on the other, and a queen appearing on move three.
Stopping one
You do not chase a passed pawn from behind, you get in front of it. The square directly ahead is the blockading square, and the best blockader is a knight, because a knight standing on d6 in front of a d5 pawn still attacks eight squares while it does the job. A king blockades well in the endgame and badly in the middlegame, where it gets attacked. A rook is the worst blockader, since it wants to be behind the pawn instead, whether the pawn is yours or theirs.
And the pawn you cannot blockade at all is a passer with a friend beside it: two connected passed pawns on the sixth rank beat a rook, because the two of them cover each other’s queening squares.
If you want to feel how fast these things move, the advanced pawn puzzles are all about a pawn one step from the eighth rank, and pawn endings are where the counting becomes the whole game. Once the pawn arrives, promotion does the rest.