Find common factors for the numerator and denominator. 12 and 40 are both even so they are multiples of 2. They are also multiples of 4. Divide both by 4 and the result is 3/10. Since 3 is a prime number this can't be reduced further.

Find the highest denominator which is 4. 4 goes into 12 is 3 and 4 goes into 40 is 10. Therefore 3/10 is your answer

12 to 3

3/8

3/10

12/40 = 3/10 = 0.3