The third proportional of two numbers 7 and 21 is
Three numbers a,b,c are in proportion iff $$b^2 = ac$$
Here, a = 7 and b = 21
=> $$(21)^2 = 7 \times c$$
=> $$c = \frac{21 \times 21}{7} = 3 \times 21$$
=> $$c = 63$$
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