The number line shows the temperature at midnight in a city. By noon the temperature has risen by \(19^\circ\)C. Write down the temperature at noon.
Question 1 Report
The number line shows the temperature at midnight in a city.
By noon the temperature has risen by \(19^\circ\)C.
Write down the temperature at noon.
Answer Details
The number line gives the midnight temperature as \(-7^\circ\)C, a point \(7\) units to the left of zero. A rise in temperature moves to the right along the line, so add the rise:
\[ -7+19=12 \]
The temperature at noon is \(12^\circ\)C. [B1]
Counting it out makes the arithmetic clear. Moving \(7\) units to the right from \(-7\) reaches \(0\), which uses up \(7\) of the \(19\) degrees. The remaining \(19-7=12\) degrees carry the temperature to \(12^\circ\)C above zero.
The common error is to add the digits and ignore the sign, giving \(26^\circ\)C, or to subtract and give \(-26^\circ\)C. A rise always moves the value in the positive direction, and here the rise is larger than the size of the starting temperature, so the result must end up above zero.
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