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JAMB 1998 · UME · Q1

If 10112+X7=25101011_2 + X_7 = 25_{10}, solve for XX.

Worked solution (try it first)
  1. Change to base ten: 10112=8+2+1=111011_2 = 8 + 2 + 1 = 11.
  2. So X7=25−11=14X_7 = 25 - 11 = 14 in base ten.
  3. Write 14 in base seven: 14=2×7+014 = 2 \times 7 + 0, so X=20X = 20, option B.

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