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$c+c=6$

or, $2c=6$

or, $c=3$ .....................[1]

now, $c+b=5$

or, $3+b=5$

or, $b=2$ .......................[2]

again, $a+b=10$

or, $a+2=10$

or, $a=8$ .......................[3]

so, $a+d=11$

or, $8+d=11$

or, $d=3$

bcz c +c = 6, so c = 3
and c + b = 5 so b = 2
and a + b = 10 so a = 8
and a +d = 11 so finally d = 3
d = 3

As $c + c = 6$ , then $c = 3$ .
We found the value of $c$ , then the equation $c + b = 5$ , we can substitute the value for $c$ , so the answer is, $3 + b = 5$ , then $b = 2$
Go to the first equation, $a + b = 10$ , then $a + 2 = 10$ , $a = 8$ .

At last, $a + d = 11$ , $8 + d = 11$ , $d = 3$