We subtract 600 from the total cost and get 1200-600=600. We have removed the difference between the cost of the individual items, so we are now left with the cost of two blouses. The cost of one blouse is therefore 600/2=Php300. The pair of pants is 600 more than this=Php900.
The question says "at least", so the blouse costs at least Php300 and the pants cost exactly Php600 more than this.