Given the expansion(a+2b+4ab)10
Tr+1=a10⋅b10(b1+a2+4)10
Tr+1=a10⋅b10C10r1+r2(b1)r1(a2)r2410−r1−r2
Tr+1=a10⋅b10r1!r2!(10−r1−r2)!10!(b1)r1(a2)r2410−r1−r2
Putting, r1=2,r2=3
Coefficient of a7b8 is 2!.3!5!10!⋅23⋅410−2−3
=2!.3!5!213⋅10!
=216⋅315
K.216=315..216
Hence, K=315