If W and P are the unit costs of watermelons and papayas then:
4W+5P=45.75, 5W+10P=66.
So 8W+10P=91.50, 10P=91.50-8W, and we can substitute in the other equation:
5W+91.50-8W=66, 3W=91.50-66=25.50, W=25.50/3=8.50, and 12 watermelons cost 12×8.50=R102.