Let the speed of the bullet be v1. Let the common velocity of the bullet and the block, after the bullet is attached like a simple pendulum is v2. By the principle of conservation of linear momentum
10
1000
×140+
100
1000
×0=
100
1000
v1+
10
1000
v2 140=10v1+v2 . . . (i) For the block, 0=v12−2g(
80
100
) ⇒v1=4m∕s ⇒140=10×4+v2 [from Eq. (i)] ⇒v2=100m∕s Final velocity of bullet =100m∕s