You use multiplication
1 pound = 0.18 dollars
10.5*(1 pound) = 10.5*(0.18 dollars) .... multiply both sides by 10.5
10.5 pounds = (10.5*0.18) dollars
10.5 pounds = 1.89 dollars
So in short you simply multiply 10.5 and 0.18 to get 1.89 dollars as the final answer