Tuesday

Raven reads the tape measure along the board. "12.75 meters for the first section. Then 8.4 meters for the next."

"12.75 plus 8.4," Rocket says, writing fast. "That is 13.59."

Raven looks at his paper. "What do you notice about where you put the 8.4?"

Rocket looks. The 8 sits under the 7, and the 4 under the 5. "I lined up the ends, not the points."

Nova hovers over the board. "A decimal point is a place marker," she says. "Points above points, always."

Rocket rewrites it. "12.75 plus 8.40. That is 21.15 meters." He grins. "Much better than 13."

"Now multiply it by four laps," says Raven. Rocket groans. Nova hums. "Count the decimal places, that is all."

Write one number under the other with the decimal points in a column. Add zeros so both have the same number of places.

12.75 + 8.40 = 21.15. The point in the answer sits right under the others.

Subtract the same way: 20.50 - 8.75 = 11.75.

Multiply as if there were no points. Then count the decimal places in both numbers and put that many in the answer.

2.5 × 1.2: first 25 × 12 = 300. One place plus one place is two places, so 3.00, which is 3.

0.6 × 0.4: first 6 × 4 = 24. Two places in all, so 0.24.

To divide by a decimal, move the point in both numbers the same number of places until the divisor is whole.

4.5 ÷ 0.5 becomes 45 ÷ 5 = 9. Moving both points keeps the quotient the same.

When the dividend is smaller, keep dividing past the point: 15 ÷ 4 = 3.75.

Points in line. Tomorrow is Numbers Lab: you measure a course of your own to the hundredth of a meter.