Break the numerator and denominator into prime numbers. Prime numbers are values that can only be divided evenly by themselves and the number 1. For example, if you have 70/90, break 70 into 7 * 5 * 2 and 90 into 3 * 3 * 5 * 2.
Strike out any common prime factors. In this example, you would strike out 5 and 2 from both the numerator and denominator.
Multiply the remaining factors in the numerator and denominator. In this example, you would only have 7 in the numerator, and you would have 3 time 3 in the denominator, which equals 9. Therefore, the fraction 7/9 would be the lowest terms for the original fraction 70/90.