Hey!
if a:b = c:d , then we write, a:b::c:d .
So, in this case, 10:990::3:x
we need to find x, by above condition
therefore, 10:990 = 3:x
means, 10/990 = 3/x
or, 1/99 = 3/x
or x = 99*3
therefore, by solving we get, x = 297
So, the missing number (x) is equal to 297.
10:990::3:297.
Hope it helps.
thankyou
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