What has many keys but can’t open a single lock?

There are 2 people near the river and both of them want to get on the other side. The boat can only take one of them. But they got across.
How?

Name one eight-letter word with 'kst' in the middle, 'in' the beginning, 'and' at the end.?

What's the next in the series?

H, Be, F, S, Mn, Kr, In, Gd, Tl, ?

John gave half of the apples he had plus one more to Jacob. He gave half of the remaining ones plus one more to James. Now, John was left with just one apple.

Can you find out how many did he have in the beginning?

An express train takes 3 seconds to enter the tunnel which is 1 km long.
If it is travelling at 120 km an hour, how long will it take to pass completely through the tunnel?

What four weights can be used to balance from 1 to 30 pounds?

What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?

John was working on a Chemical Mixture whose weight comprise 90% liquid and 10% solid. The total weight of the mixture is 20 pounds. After a while, John noticed that some of the liquid evaporated and now the liquid comprises just 50% of the weight.

What is the weight of the chemical mixture now?

What does below science rebus mean?

Ag Ag Ag
Ag SPOON Ag
Ag Ag Ag