Algebra · real student question

Solve 0.85x x 1.09 x 0.85 = 3250000/2.

Question

Solve 0.85x1.090.85=325000020.85x \cdot 1.09 \cdot 0.85 = \dfrac{3250000}{2} for xx.

Step-by-step solution

  1. Simplify the right-hand side. 3,250,0002=1,625,000\dfrac{3{,}250{,}000}{2} = 1{,}625{,}000, so the equation is 0.85x1.090.85=1,625,0000.85x \cdot 1.09 \cdot 0.85 = 1{,}625{,}000.

  2. Collapse the multiplier chain on the left. All three decimals multiply xx, so combine them into a single coefficient: 0.851.09=0.9265,0.92650.85=0.787525.0.85 \cdot 1.09 = 0.9265, \qquad 0.9265 \cdot 0.85 = 0.787525. Reading the chain in business terms: two 15%15\% reductions and one 9%9\% increase leave 78.7525%78.7525\% of the original.

  3. Rewrite as a one-step linear equation. 0.787525x=1,625,000.0.787525x = 1{,}625{,}000. Only now is a single division enough to finish.

  4. Divide, keeping full precision. x=1,625,0000.787525=65,000,000,00031,5012,063,426.56.x = \frac{1{,}625{,}000}{0.787525} = \frac{65{,}000{,}000{,}000}{31{,}501} \approx 2{,}063{,}426.56. The exact value is the fraction 6500000000031501\tfrac{65000000000}{31501}; rounding 0.7875250.787525 to 0.78750.7875 first would shift the answer by about 6565, so do not truncate the coefficient.

  5. Verify by running the chain forward. 2,063,426.5579×0.85=1,753,912.5742{,}063{,}426.5579 \times 0.85 = 1{,}753{,}912.574, ×1.09=1,911,764.706\times 1.09 = 1{,}911{,}764.706, ×0.85=1,625,000.00\times 0.85 = 1{,}625{,}000.00 - the target is recovered.

  6. Sanity-check the magnitude. Since the net multiplier 0.7875250.787525 is less than 11, xx must be larger than 1,625,0001{,}625{,}000; the ratio 1/0.7875251.26981/0.787525 \approx 1.2698 predicts about 2.062.06 million, which matches.

Answer

x=16250000.787525=65000000000315012,063,426.56x = \frac{1625000}{0.787525} = \frac{65000000000}{31501} \approx 2{,}063{,}426.56

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