The expression is 
Place value chart for 5.8 and 4.2.
\
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5 \ | \
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8 \ | \
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4 \ | \
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2 \ | \
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Line up the decimal points and draw a line below the numbers.
\
First find the sum of the tenths digit.
\The sum of the tenths digit
.

Next find the sum of the ones digit.
\The sum of the ones digit
.

The sum is 10.