May i know why + (15-8). Where the 15 and 8comes from

Good question. Part (b) asks what is the difference in the number of sticks used between figure 9 and 11, correct? I believe you know why it is 5+3=8? So every odd figure number to the next, the difference is always 8. The same goes for the difference between two consecutive even numbers.

Now, 30 is the 15th even number, which is why I divided 30 by 2. 8 is the difference between two consecutive even figure numbers. The first even figure number is 15 which is like adding 7 (from the 0th even number) to an 8 (to promote to the 1st even number).

So....
Figure number 2 is 7+8=15
Figure number 4 is 15+8=23
Figure number 6 is 23+8=31
Hope you get the idea!

Let me know if you are still confused.

Each time you jump two figures, the number of sticks increases by 8.

You can try to start from Figure 2 also
(which has 15 sticks)

From Figure 2 to Figure 30,

Number of figures jumped = 30 - 2 = 28

Since each jump is two figures,

Number of jumps = 28 ÷ 2 = 14

Increase in number of sticks = 14 × 8 = 112

Number of sticks at figure 30
= 15 + 112
= 127

If you start from another even numbered figure (eg. Figure 4, which has 23 sticks)

From Figure 4 to Figure 30,
Number of figures jumped = 30 - 4 = 26

Since each jump is two figures,
Number of jumps = 26 ÷ 2 = 13

Increase in number of sticks = 13 × 8 = 104

Number of sticks at figure 30
= 23 + 104
= 127


If you start from Figure 6 : (which has 31 sticks)

From Figure 6 to Figure 30,

Number of figures jumped = 30 - 6 = 24

Since each jump is two figures,

Number of jumps = 24 ÷ 2 = 12

Increase in number of sticks = 12 × 8 = 96

Number of sticks at figure 30
= 31 + 96
= 127