1/18-9/20

Let's evaluate: $\frac{1}{18}-\frac{9}{20}$

ⓘ To add fractions, put them over the same denominator, then add the numerators. We choose $180$, the least common multiple of $18$ and $20$. Multiplying the numerator and the denominator by the same number does not change the fraction: $\frac{1}{18}-\frac{9}{20}$ = $\frac{1\times 10}{18\times 10}-\frac{9\times 9}{20\times 9}$ = $\frac{1\times 10-9\times 9}{18\times 10}$ = $\frac{10-9\times 9}{18\times 10}$

$\frac{10-9\times 9}{18\times 10}$

ⓘ Multiplications are done before additions and subtractions: $9\times 9$ = $81$

ⅰ $18\times 10$ = $(10+8)\times 10$ = $10\times 10+8\times 10$ = $100+80$ = $180$

$\frac{10-81}{180}$

ⓘ The numerator and the denominator are computed before the division: $10-81$ = $-71$

▶Result:   $-\frac{71}{180}$

The numeric value of: $-\frac{71}{180}$ equals:

▷ $\approx -0.3944$