the part of mathematics that involves the adding and multiplying, etc. of numbers

I've never been very good at arithmetic. 我从来不擅长算术。

an arithmetic test 算术考试

calculations involving adding and multiplying, etc. numbers

I did some quick mental arithmetic and decided it was going to cost too much. 我快速心算了一下，觉得费用会太高。

I can't figure out which of these is cheaper - could you do the arithmetic? 我弄不清楚哪种便宜些，你能算一算吗？