Question

The cardinal number of $A=\left\{\right(x:x=5n,n\in W,n<4\}$ is ___.$\left[2Marks\right]$

Answer 1

Given the set $A=\left\{\right(x:x=5n,n\in W,n<4\}.$$\left[0.5Mark\right]$

$n$ can take values of 0, 1, 2 and 3$\left[0.5Mark\right]$

Thus, the elements of A are $5\times 0,5\times 1,5\times 2$ and $5\times 3.$

$\therefore A=\{0,5,10,15\}$$\left[0.5Mark\right]$

$\therefore n\left(A\right)=4$$\left[0.5Mark\right]$