Question

Read the numbers and decide what the next number should be. 5 15 6 18 7 21 8

Answer 1

we have

$5,15 \\6, 18 \\7,21 \\8,?$

Let

$a1=5\\b1=15\\a2=3\\b2=18\\a3=7\\b3=21\\a4=8$

we know that

$\frac{b1}{a1}=\frac{15}{5}\\ \\b1=3*a1$

$\frac{b2}{a2}=\frac{18}{6}\\ \\b2=3*a2$

$\frac{b3}{a3}=\frac{21}{7}\\ \\b3=3*a3$

$a4=8$

so

the next term should be

$b4=3*a4=3*8=24$

therefore

the answer is

$24$

Answer 2

24 9 27 10 30 ..... n followed by n³ then followed with n+1