What you could try to do (besides using a calculator) is try to get the denominator to 100 and go from there. For example, the fraction 4/20. We multiply both sides by 5, giving us 20/100. Then, we move the decimal over twice, giving us 0.2.
If Chen has 4 pizzas and you need to know in terms of thirds the logical thing to do would be to set 4 to a fraction over 3 (12/3). Since each friend will eat 2/3 of a pizza you divide 12 by 2 (taken from the numerators). This will get you to 6.
