4% of 25 is 1 so we know it is only a bit more than 25 dollars for the original dinner. 4% of 30 is 1.2 so we know we are close. If we go a little higher, to 31, it is 1.24, and calculating 4% of 32 is 1.28, too much. We could try 31.5, but that is 1.26, too much. Trying with 31.25 gets us to 1.25. This is the right answer but we still need a tip. So we take the decimal point in 32.50, the total bill, and move it one place to the left, giving us 3.25, which is 10% of the bill. However, we need 20% so we should multiply 3.25 by 2, which is 6.50. 6.50 is the tip, so we should add that to the cost. This is 32.50+6.50, or $39.00. All in all, Hannah's meal cost 39 dollars.
