IMAS 2020 — Upper Primary Grades 5–6 — Round 2 Question 8

Using some number of black and white balls of the same size, we arrange the figures as shown in the diagram below. If Figure 1 uses 8 balls, Figure 2 uses 13 balls, Figure 3 uses 20 balls and so on. How many balls are there in Figure 10?









Figure 1:

Figure 2:

Figure 3:

Figure 4: