One 10 BTC input, one 10 BTC output to your friend. He then returns 7 BTC to you in a later transaction.

One 10 BTC input, one 10 BTC output to your friend. And then he returns you 7 BTC in next transaction

One 10 BTC input, two outputs: 3 BTC to your friend and 7 BTC back to yourself.

One 3 BTC output to your friend. The rest stays in your address.

Solving complex mathematical equations the protocol asks them to solve

Hashing the block header with different nonces, looking for one below the target

Validating pending transactions. The first miner to finish produces the block.

Negotiating with other miners about which block should come next

Block times stay at ten minutes because the protocol immediately retargets

Nothing changes because miners coordinate to keep block times stable

Block times increase to about 20 minutes until the next retarget.

Block times drop to about 5 minutes until the next retarget

Transaction fees paid by users will be the only source of miner revenue

Bitcoin will stop producing new blocks once all bitcoins are mined.

The chain will be hard-forked to increase supply and keep producing block rewards.

A reserve fund built up from earlier years will continue paying them

Both transactions are included in the next block, and the protocol picks a winner

Only one is included in a block. The other is rejected once the first confirms

The network rejects both because of the conflict

The electricity cost is what makes attacking the network too expensive to be profitable

Energy use is a side effect that the protocol would eliminate if it could

The protocol requires a minimum number of nodes to maintain the 10-minute block time.

Mining hardware needs cooling to function at peak performance

Yes, but it would compromise the properties Bitcoin was designed to protect

Yes, Bitcoin is moving in that direction right now.

No, the cryptography Bitcoin uses cannot support that throughput

Yes, the technology already exists but hasn't been deployed