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Word Problems Involving Decimals

When you get a word problem that involves adding or subtracting decimals , it's usually a good idea to rewrite all the numbers with the same number of decimal places, so you don't get confused.

Example 1:

Joey, Keith, and Eli have a combined height of $7$ meters. If Joey is $2.31$ meters tall and Eli is $2.6$ meters tall, how tall is Keith?

First, rewrite all the numbers with the same number of decimal places.

Joey, Keith, and Eli have a combined height of $7.00$ meters. If Joey is $2.31$ meters tall and Eli is $2.60$ meters tall, how tall is Keith?

Now write the equation.

$2.31 + 2.60 + k = 7.00$ where $k$ is the height of Keith in meters.

Combine like terms.

$4.91 + k = 7.00$

Subtract $4.91$ from each side.

$k = 2.09$

Therefore, Keith is $2.09$ meters tall.

This rewriting with the same number of digits may not be so important if the problem requires you to multiply decimals or divide them. But in any case, you should check your answer at the end to make sure it makes sense. A small mistake can cause your answer to be off by a factor of ten or one hundred... or worse!

Example 2:

James works at an Indian sweet shop. He needs to fill boxes with $0.3$ kilograms of coconut barfi each. If he has $8$ kilograms of coconut barfi, how many boxes can he fill?

This is a division problem: we need to find how many times $0.3$ kilograms goes into $8$ kilograms.

$8 \div 0.3 = 26. \bar{6}$

Since the question asks for the number of boxes he can fill , the decimal part of the answer can be ignored. James can fill $26$ boxes, with a little bit of barfi left over.