0.09 = (1/10) × what
Multiply both sides by 10.
0.90 = 10/10 × what
0.90 = what
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The first step is to write the relationship using mathematical symbols. (The math sentence follows the English sentence very closely, so that should not be trouble for you.)
The second step is to identify the coefficient of the variable (1/10) and then multiply both sides of the equation by its inverse (10/1). This leaves the variable multiplied by 1 and equated to the solution value.
0.09 is 1/10 of 0.90
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You can also recognize that 1/10 of something can be found by moving the decimal point one place to the left. That is, 1/10 of 12.34 is 1.234. This is a place-value thing.
Since 0.09 is 1/10 of something, it must have had its decimal point moved 1 place to the left. To find the original something, we move the decimal point back 1 place to the right: 0.9.
