#### Answer

The number we are looking for is 350.

#### Work Step by Step

Let X represent the number we are looking for.
1.2% of what number means 1.2% $\times$ X
So, this question translates to:
1.2% $\times$ X = 4.2
0.012 $\times$ X = 4.2
Divide both sides by 0.012
X = 350