A ladder hs 10 steps. A labour has to carry some weight to the top. He climbs two steps up in one second and 1 step down in nxt scond by dancing in a famous tune. In how many seconds he will be at top?

Since two steps climbed by labour in one second and 1 step down in next second
So, net steps climbed by the labour in 2 seconds = $2-1 = 1$ step
So to climb 1 step a labour takes 2 seconds.
Therefore to climb 10 steps, time taken by labour = $10\times 2$ = 20 seconds
Therefore he will be at top after 20 seconds.