#### Answer

The number we are looking for is 500.

#### Work Step by Step

Let X represent the number we are looking for.
2.1% of what number means 2.1% $\times$ X
Is 10.5 means =10.5
So, this question translates to:
2.1% $\times$ X = 10.5
0.021 $\times$ X = 10.5
Divide both sides by 0.021
X = 500