For a barrel of oil, three tenths of it is used for the first time, and 10kg is used for the second time. Two fifths of this barrel of oil is used for the two times, so it's not enough Because the title couldn't be written down, I went on to write it down How many kilos of barrel oil are there? How many kilos are left after two times of use Please write the formula

For a barrel of oil, three tenths of it is used for the first time, and 10kg is used for the second time. Two fifths of this barrel of oil is used for the two times, so it's not enough Because the title couldn't be written down, I went on to write it down How many kilos of barrel oil are there? How many kilos are left after two times of use Please write the formula

10 (2 / 5-3 / 10) = 100 kg
So it's 100 kg in all
There is 100 * (1-2 / 5) = 60 kg left

After 15 kg of oil is used in a barrel, the area used is 3 kg more than the remaining two fifths. How many kg of oil is there in this barrel?
Math problem, 1 minute speed~~

(15-3) / (2 / 5) = 30 (kg)
15 + 30 = 45 (kg)
A: this barrel of oil used to be 45 kg

A bucket of water, using 25 of it, has 15 kg left. How many kg does this bucket of water weigh?

A: this bucket of water weighs 25 kg

After two fifths of a barrel of oil has been used up, there are two fifths of a kilogram left, which is () kilogram
Formula calculation!
5 grams of salt is dissolved in 20 grams of water, the ratio of salt to water is (), and the salt content of brine is ()

2/5÷（1-2/5)=2/3
5:20=1:4
5/(20+5)=20%