Given,
f(1)+f(2)+1=f(4)
⇒f(1)+f(2)≤5 asf(4)≤6
Now taking cases we get,
Case (1),f(1)=1⇒f(2)=1,2,3,4→4mappings
Case (2),f(1)=2⇒f(2)=1,2,3→3mappings
Case (3),f(1)=3⇒f(2)=1,2→2mappings
Case (4),f(1)=4⇒f(2)=1→1mappings
And f(5)&f(6)\rightarrow 6\text{mapping each}
So, total number of function will be =(4+3+2+1)×6×6=360