Inez waters her plants every two days she trim them every 15 days she did both today when will she do both again

She will do both again in 30 days. 

She waters every two days and 2 is a even number.

She trims her plants every 15 days and 15 is an odd number.

When you add two odd numbers you get an even 15 + 15 = 30

2 × 15 = 30