random

Question 1

trade-off
1.trade something off (against/for something)
​to balance two things or situations that are opposed to each other
They were attempting to trade off inflation against unemployment.
N
1.trade-off (between something and something) the act of balancing two things that are opposed to each other
a trade-off between increased production and a reduction in quality
There is a trade-off between the benefits of the drug and the risk of side effects.

Question 2

stumble upon
1.stumble across/on/upon something/somebody
​to discover something/somebody unexpectedly
Police have stumbled across a huge drugs ring.
I stumbled across Thompson outside the hotel.
I stumbled across the place quite by chance.

Question 3

while away
1.while something away
​to spend time in a pleasant lazy way
We whiled away the time reading and playing cards.