A student computes 3.7 + 2.45 by right-aligning the digits (stacking the 5 under the 7). What is wrong with this approach?
ANothing is wrong — right-aligning always works for addition.
BIt adds tenths to hundredths instead of matching each place value to its equivalent.
CIt produces an answer that is too small by exactly 1.
DIt only fails when one number has more digits than the other.
Right-aligning stacks the 5 (hundredths) under the 7 (tenths), treating them as the same place value. This is like adding dimes and pennies without converting. Aligning the decimal points ensures each column contains digits from the same place value.
Question 2 True / False
Rewriting 3.7 as 3.70 before adding changes the value of the number, so you is expected to adjust your final answer to compensate.
TTrue
FFalse
Answer: False
Appending a trailing zero after the decimal point does not change a number's value — 3.7 and 3.70 are identical. Trailing zeros to the right of the last significant decimal digit are placeholders that make the column structure explicit without altering the quantity.
Question 3 Short Answer
Why does lining up decimal points work as the rule for decimal addition, rather than some other alignment strategy?
Think about your answer, then reveal below.
Model answer: Lining up decimal points aligns every digit with its correct place value (tenths with tenths, hundredths with hundredths, etc.), so you are always adding like units — the same reason whole-number addition requires aligning ones with ones.
The decimal point marks the boundary between whole-number and fractional place values. Aligning the points automatically lines up all place values on both sides. Any other alignment would mix place values, producing a wrong answer for the same reason that adding 30 + 4 by right-aligning works but adding 3.0 + 0.4 by right-aligning would not.