Question 42102
Let there be 'x' robbins and 'y' blackbirds.
As the ratio of robbins to blackbirds is 4:7, so
{{{x/y=4/7}}}
or {{{y=7x/4}}}


Also total no. of birds = no. of robbins + no. of blackbirds = x + y = 123 (given).


Thus we have two unknowns (x & y) and two equations to solve.
{{{y=7x/4}}} ______(1)
{{{x+y=123}}}______(2)


Substituting the value of 'y' from (1) in (2),
{{{x+7x/4=123}}}
or {{{11x/4 = 123}}}
or {{{x=123*4/11}}}
or x = a fraction.


The number of birds cannot be fraction they have to be positive integers.
So, your question has erroneous data.
Before putting your question, be careful about typing mistakes in future.