These problems are very confusing. I had one too. My math book has this example if it will help:
Amelia takes a taxi from the airport to a hotel. The fare is $31.50. Suppose she wants to tip the driver 15%. What would be a reasonable amount of tip for the driver?
$31.50 is about $32
15% = 10% + 5%
10% of $32 is $3.20 Move the decimal point 1 place to the left
5% of $32 is $1.60
So, 15% is about 3.20 + 1.60 or $4.80
A reasonable amount for the tip would be $5. Hope this helps. I'm stuck on a similar problem, so I can't solve it. :(