20% of 140 is 28, and 10% of 28 is 2.8 which I'm assuming you would round up to 3. 28+3=31. so you were called back 31 times after your 140 customers:)
A method to solve it would be would be: First converting 5% to a decimal then multiplying that by 15.80 in order the find 5 percent of 15.80...Afterwards you have to add that value to the original 15.80 ..And you should end up with the same answer.