Answer
3 : 7
Explanation
A ratio is a fraction.
36 : 84 ⇒ 36/84
Simply this fraction by dividing numerator and denominator by 4
(36 ÷4) / (84 ÷ 4) = 9/21
Divide by 3 on both numerator and denominator.
(9 ÷ 3) / (21 ÷ 3) = 3/7
3/7 = 3 : 7