Figure 1: difference between ∆ and O
= 2 - 2 = 0

Figure 2: difference = 5 - 4 = 1

Figure 3: difference = 9 - 7 = 2

Figure 4: difference = 14 - 11 = 3

Figure 5: difference = 20 - 16 = 4

Notice that the difference is always 1 less than the figure number.

So if the difference is 50, the figure is Figure 51.


To find out the total no. of ∆ and O in Figure 51, you should have solved part b this way:

Total for figure 1 = 4 = 2 x 2
Total for figure 2 = 9 = 3 x 3
Total for figure 3 = 16 = 4 x 4
Total for figure 4 = 25 = 5 x 5
Total for figure 5 = 36 = 6 x 6

Notice that the total for the figure

= (Figure no. + 1 ) x (Figure no. + 1)

For part b), total = 100
= 10 x 10
= (9 + 1) x (9 + 1)

The figure number is 9.

So for part c), total in figure 51

= 52 x 52 = 2704