Question

a 60 litre tank was full of oil. sam used 40% of it and poured the rest into a 40 litre tank. what percent of the 40 litre was filled with oil.

Answer 1

after using 40 % Sam had
$60 - (60 \times \frac{40}{100} ) \\ = 60 - 24 = 36$ lit

now he poured it into a 40 lit tank.
so now the 40 lit tank was filled with
$(\frac{36}{40} \times 100) = 90$ % oil