The original area of the triangle is (18 * 5) / 2 = 45
18 cm decreases by 30% = 18 - 0.30(18) = 12.6
5 cm increased by 20% = 5 + 0.20(5) = 6
so the new area is (12.6 * 6)/ 2 = 37.8
so there is a percent decrease
percent decrease = (original number - new number) / original number....x 100
(45 - 37.8) / 45 = 7.2 / 45 = 0.16.....x 100 = 16%..so there is a 16% decrease