Answer is N = 20
Step by step
1/4 of a number means 1/4n
Is removed from is subtract
2/5 of that number means 2/5n
Result is 3 means equal = 3
Put this into an equation
2/5n - 1/4n = 3
Our common denominator is 20
2/5 = 8/20 ( multiplying top and bottom by 4)
1/4 = 5/20 (multiply top and bottom by 5)
8/20n - 5/20n = 3
Subtract
3/20n = 3
Divide both sides by 3/20 to solve for n
When we divide fractions we flip the divisor and multiply
3/20 * 20/3 n = 3 * 20/3
Multiply
n = 20