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Answer:
D option
Step-by-step explanation:
a + b = 21
Placing value of b as 3/4a,
a + 3/4a = 21
7a/4 = 21
a = 12
Putting value of a in b=3/4a,
b = 3 × 12/4 = 9
a = 12, b = 9
Hope it helps.