Explain how an estimate you place the decimal point when multiplying 3.7 times 5.1

Let's multiply 3.7 times 5.1.

             5.1
         x  3.7
       
         3 5 7
  +  1 5 3
 
      1 8 8 7

We multiply 3.7 by 5.1 and get 1887 before we deal with  the decimal point.
Is the answer 0.1887, 1.887, 18.87, 188.7 or 1,887?

Now we use an estimate to figure out where to put the decimal point.
3.7 is close to 4.
5.1 is close to 5.
4 * 5 = 20, so 3.7 * 5.1 must also be close to 20. The product cannot be 188.7 or 1,887. It must be 18.87 which is the only number using the digits 1, 8, 8, 7 in that order that is close to 20.