Which of the following steps could be used as a shortcut method to multiply 12 by 50

Multiply 12 by 100, then divide that result by 2

I don't think you have attached the options, but I'll show you a shortcut method to solve this anyway (which may or may not be in the multiple choice answers):

1. Compute 12*5 
2. Add a zero at the end of the product

Another way (which is slightly longer):

1. Split 50 into 10*5
2. Multiply 12*5 to get 60
3. Multiply 60*10 to get 600

Where is the problem