Question

42 is 21 % of what number

Answer 1

$21\%=0.21\\\\ 0.21x=42\\ \boxed{x=200}$

Answer 2

X - the number

$21\%x=42 \\ \frac{21}{100}x=42 \\ x=42 \times \frac{100}{21} \\ x=2 \times 100 \\ x=200$

42 is 21% of 200.