Answer:
30% also means 30/100 in fraction form.
(Think of the root 'per' in percent to be per a 100.)
The keyword 'of' means multiplication.
'X' will be the unknown number aka the part that says ('-what number').
And 123 will represent the result.
Set up equation;
30/100 · x = 123
Then solve for x;
30/100x = 123
0.3x = 123
÷ 0.3 ÷ 0.3
x = 410
So your answer is 410.