Big Dig Diamond Company pulverizes 156 tons of rock for every 2 ounces of diamonds it finds. How many tons of rock must it grind up in order to locate 20 ounces of diamonds?

 You could do it as 2*10=20 so you will have multiply 156 by 10, getting 1,560. The other way is by cross multilying the fraction 2/156 by 20/? and dividing the answer; 156*20=3,120/2=1,560.