There are two piles of coal. A is 94 tons, B is 138 tons. Each day 9 tons are transported away. After a few days, the remaining coal of B is three times that of A

There are two piles of coal. A is 94 tons, B is 138 tons. Each day 9 tons are transported away. After a few days, the remaining coal of B is three times that of A

Set X days later
3×(94-9X)=138-9X
282-27X=138-9X
144=36X
X=4
A: four days later

There are two piles of coal, 94 tons in pile a and 138 tons in pile B. 90 tons each day. How many days later, the remaining coal in pile B is three times that in pile a?

After X days, the remaining coal in pile B is three times that in pile a
A had (94-90x) days left after X days; B had (138-90x) days left after X days
138-90x=3*(94-90x)
X = 0.8 is about 1

There are 5400 tons of coal in a pile. In the first month, two fifths of the coal is used, and in the second month, four fifths of the coal is used. How many tons are left?

5400 × (1-2 / 5-4 / 9) = 840 tons

5. There are 5400 tons of coal in a pile. Two fifths of the coal is used in the first month and four ninths in the second month. How many tons are left?

5400×（1-2/5-4/9）
=5400×7/45
=840 tons
A: there are 840 tons left