Hey there! :)
(1) Change to decimal and multiply by the number you need to find the % of
75% as a decimal = 0.75
0.75 × 28 = 21
Your answer is 21
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(2) Circumference = π · diameter
To find diameter, divide circumference by π
Circumference = 37.68
37.68 ÷ π = 11.99 = 12
The diameter is 12
Hope this helps :)