It seems like they're both divisible by 8, so let's do that.
40/64 = 5/8
There is no way to divide 5/8 while keeping the whole numbers.
Therefore, 40/64 has an equivalent fraction of 5/8.
However, that is the simplest equivalent fraction. Other ones can include:
40/64 = 80/128 (multiply both sides by 2)
40/64 = 20/32 (divide both sides by 2)
As long as you only multiplied or divided both the numerator and the denominator with same values, it is considered as an equivalent fraction as the fraction before