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SASMO 2015 P2 Q7  Study the figures made with matchsticks below. How many matchsticks are needed to make figure 5? 
Overseas Competition->Knowledge Point->Counting Modules->Counting the Number of Figures->Classifying and Enumerating->Counting Regular Figures->Counting Triangles
Figure 1 = 1 triangle = 3 matchsticks  Figure 2 = (1+2) triangle = 3 x 3 = 9 matchsticss.  Figure 3 = (1+2+3) triangle = 6 x 3 = 18 matchsticks.  Figure 4 =  Figure 5 = (1+2+3+4+5) triangle = 15 x 3 = 45 matchsticks. 