Hi, yes mathematics will be included in best 4 for DU admissions. and yes student who took up applied sciences is eligible for B. Com (//B.Com) hons and ECO hons. Every university or college accepts mathematics for B.Com Oxford university accepts too. Below is the link to an article about eligibility criteria for B.Com (//B.Com) hons. Maths is compulsory for any college in DU for economics. For B.Com (//B.Com) you need to have 50% + in your board exams. Hope this helps.

https://university.careers360.com/articles/du-bcom-eligibility-criteria