Answer: 1.4 + (-1.7) = -0.3
See the diagram below for the number line graph
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Step-by-step explanation
Draw out a number line. Or use a number line that your teacher has given you. Plot 1.4 on it. This number is between 1 and 2. Mark out 10 equal spaces between 1 and 2. Then move 4 spaces to the right to go from 1 to 1.4; refer to the diagram below.
Once you determine where 1.4 is, move 1.4 units to the left to arrive at 0. Move another 0.3 units left to arrive at -0.3; notice how you have moved 1.4+0.3 = 1.7 units total to the left.
This shows us that 1.4 + (-1.7) = -0.3
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Another way to think about it: you start on the 14th floor of a building. Move down 14 floors to get to the ground floor (aka floor 0). Move down another 3 floors to get to basement level 3, in which we denote as -3. In total you have moved down 17 floors (since 14+3 = 17).
This signifies that 14-17 = 14 + (-17) = -3. Then stick decimal points in each value to get 1.4 + (-1.7) = -0.3