word problem with addition of 3 or 4 decimals and whole numbers

Question

 Juan filled his gas tank four times last week. Here are the amounts of gas he bought (in gallons).

 14.1, 4, 18.35, 3.93

What is the total amount of gas Juan bought last week?

Answer 1

easiest method is plug each number into a calculator, even the cheapest, most simple calculator has an addition function

But you can do it by hand

14.1+ 4 = 18.1

18.35 + 3.93 = 22.28

18.1 + 22.28 = 40.38 gallons

just to check, use the "rule of 9's" sum of digits in the problem should = sum of digits in the answer

14.1 sum to 6

6+4 = 10, 1+0 = 1

18.35 sum to 17, 1+7 = 8

3.93 sum to 15 1+5 =6

1+8+6 = 15, 1+5 = 6

the solution 40.38 sums to 15, 1+5 = 6

that's not an ironclad guarantee it's right, but about 90% chance you didn't make a mistake somewhere.

or round off each number, 14+4 +18+ 4 = 40. That at least shows you're in the right range for the answer