What does this rebus riddle means ?

Replace the question mark with the number below given a simple math problem.

I am weightless, but you can see me. Put me in a bucket, and I'll make it lighter. What am I?

Name a key which is hardest to turn.

A car is crossing a 20 km-long bridge. The bridge can support at most 1500kg of weight over it. If somehow, the weight on the bridge becomes more than that, it will break.

Now, the weight of the car is exactly 1500kg. At the midway, a bird comes and sits on the roof of the car. This bird weighs exactly 200 grams.
Can you tell if the bridge breaks at this point or not?

What breaks yet never falls, and what falls yet never breaks?

I am beautiful, up in the sky. I am magical, yet I cannot fly. To people I bring luck, and to some people, riches. The boy at my end does whatever he wishes. What am I?

Can you decipher the following common phrase?

T M C
A U O
H S M
W T E

A pond had 60 fish, out of which, all but 20 died.
How many fishes are now present in that pond?

I have Rs.50 and I spend it in the way as below.