Question

100×10-(100-2×x-60)×10=900

Answer 1

$100 \times 10 - (100 - 2 \times x - 60) \times 10 = 900 \: | \div 10$

$100 - (100 - 2x - 60) = 90$

$100 - 100 + 2x + 60 = 90$

$2x = 90 - 60$

$2x = 30 \: | \div 2$

$x = 15$