He is short, so he can't reach the 100th floor button. On rainy days, he can use his umbrella to poke the button.
Fill the 3-gallon container with oil and pour it into the 5-gallon container. Then fill the 3-gallon container again and use it to fill the 5-gallon container the rest of the way. Out of the 3-gallon oil, 2-gallon will be required to fill the 5-gallon container completely. Hence one gallon will be left in the 3-gallon container.
Five. There are only four colors, so five socks guarantee that two will be the same color.