Complex sentence: In water of 10 C °, we would accumulate 10 ATU per day, after 13 days we would have 130 ATUs, if the water was five C °, in 13 days we would have 65 ATUs.
Split into simpler sentences: In water of 10 C °, we would accumulate 10 ATU per day; after 13 days we would have 130 ATUs. If the water was at 5 C °, after 13 days we would have 65 ATUs.