Create new value based on equation in R

Question

I am new to R so I am struggling to figure out how to do this. I have a data frame with thousands of observations that looks like this

x    total  subgroup1  subgroup2
.03  1000     500       500
.04  1050     550       500
.07  1100     525       575
.08  9900     4900      5000

I am trying to come up with a weighted value for x based on the following equation.

weighted x for subgroup1 = (Σ x*subgroup1) / (Σ subgroup1)

The idea is that for each observation we multiply the x value by the subgroup 1 value and add them all together and divide it by the sum of all the subgroup values. I also want to do this for subgroup 2. I imagine I will need some type of loop function. However, I am not sure where to start in R. Any leads would be great.