-8.1
Step-by-step explanation:

<em>Times </em><em>all </em><em>by </em><em>9</em><em> </em><em>to </em><em>get </em><em>rid </em><em>of </em><em>fraction</em>
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<em>Take </em><em>2</em><em>1</em><em>.</em><em>6</em><em> </em><em>away</em><em> </em><em>from</em><em> </em><em>both</em><em> </em><em>sides</em>
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<em>Divide</em><em> </em><em>by </em><em>-</em><em>4</em>
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I got 9.8 but if you want to round it you get 10 because you add 106 and 9 to get 115 and then add 3 and get 118 and then divide 12b and 118 and get 9.8
So, Angelique took away a a number from the two numbers and did not replace it with the correct number. So, this should be 1-51=50. Hope I helped! :)
Line 1 because the question says “less than 4”