Question

21 is what percent of 40

Answer 1

To solve for this, set up a proportion.
21/40 = x/100.
Cross multiply the fraction's numerators with the denominators; 
21 x 100, 40 and x.
2100/40x.
Divide both numbers by 40 to get x.
2100/40 = 52.5
21 is 52.5% of 40.
I hope this helps!