Equivalent ratios are created by multiplying both numbers in the ratio by the same value. In the first ratio, 15 = 5 x 3, but 10 ≠ 4 x 3, so it's not equivalent to 4:5. In 4/10, 4 = 4 x 1, but 10 = 5 x 2, so again, the two aren't equivalent.
In 16/20, though, we find that 16 = 4 x 4 and 20 = 5 x 4. Since both the 4 and the 5 are being scaled up by the same factor, 16/20 is equivalent to 4:5.
