Precedence

From tightest to loosest. Attaching a unit to a number binds tighter than any operator, so 10 m / 2 s divides two quantities.

Level	Operators
20	^, prefix -, not
16	a number and its unit
14	%, !, two quantities side by side (5 ft 11 inch)
13	*, /, mod, of, per, times, at, nearest
12	+, -, to, as, plus, minus
10	in
8	==, !=, <, <=, >, >=, before, after, ago, hence
6	and
4	or

to sits with + and -, so a to USD + b to USD does not do what it looks like. Parenthesise each conversion, or convert the sum: (a + b) to USD.