Hello.
First of all, let the number be t.
8 times t can be written like this:
Notice that the number is before the variable and the times sign (×) is not there.
We can write it as 8×t, but we usually write it as 8t.
The sum of means we add:
8t+21
Now, "less than" is "<" :
Therefore, the answer is
I hope it helps.
Have a nice day.
