Add decimals: FAQ

Estimating with adding decimals means finding an approximate answer by rounding the decimals to the nearest whole number, tenth, or hundredth.

For example, if we want to estimate the sum of $12.34$12, point, 34 and $5.67$5, point, 67, we can round both numbers to the nearest tenth: $12.34 \approx 12.3$12, point, 34, approximately equals, 12, point, 3 and $5.67 \approx 5.7$5, point, 67, approximately equals, 5, point, 7. Then, we can add the rounded numbers: $12.3 + 5.7 = 18$12, point, 3, plus, 5, point, 7, equals, 18. Estimating can help us check if our answer is reasonable or if we made a mistake.

Try it yourself with this exercise:

We can add decimals visually using grids. A grid is a square that is divided into smaller squares. We can use the grid to show how much of a whole unit we have.

For example, if we want to add $0.4$0, point, 4 and $0.3$0, point, 3, we can use a grid that has $10$10 squares in each row and column. Each square represents one-tenth or $0.1$0, point, 1. To show $0.4$0, point, 4, we can shade $4$4 squares in one row. To show $0.3$0, point, 3, we can shade $3$3 squares in another row. To find the sum, we can count how many squares are shaded in total. We have $7$7 shaded squares, which means the sum is $0.7$0, point, 7.

Try it yourself with this exercise:

To add decimals and whole numbers that have different number of decimal places, we can use zeros as placeholders to make them have the same number of decimal places. Then, we can use a place value chart to add each place value column.

Try it yourself with this exercise:

We use adding decimals in the real world whenever we deal with money, measurements, or fractions of things. For example, if we want to buy a book that costs $7.99$7, point, 99 and a pencil that costs $0.25$0, point, 25, we need to add the decimals to find the total cost. We get $7.99 + 0.25 = 8.24$7, point, 99, plus, 0, point, 25, equals, 8, point, 24. Another example is if we want to measure the length of a table that is $1.2$1, point, 2 meters long and a chair that is $0.65$0, point, 65 meters long. We need to add the decimals to find the total length. We get $1.2 + 0.65 = 1.85$1, point, 2, plus, 0, point, 65, equals, 1, point, 85.