$\"\"$

21 is compared the 50. So, 21 is the part and 50 is the whole.

$\"\"$ (Percent proportion)

$\"\"$

Apply cross multiplication property: $\"\"$ , when $\"\"$ .

$\"\"$

$\"\"$ (Multiply: $\"\"$ ) $\"\"$

$\"\"$ (Divide each side by 50)

$\"\"$ (Cancel common terms)

$\"\"$ (Divide: $\"\"$ ) $\"\"$

The 21 is $\"\"$ of 64.