options(scipen = 999) # avoid scientific notation
library(dplyr)        # request library
set.seed(123)         # same numbers everytime
dataset <- data.frame(group = c(rep(0, 5), rep(1, 5)),
                      math = c(rnorm(5,8,1), rnorm(5,3,2)),
                      reading = c(rnorm(5,7,1), rnorm(5,4,2)))
dataset[] <-round(dataset[], 1)  # round numbers

# regression model full
mod <- lm(math ~ reading + group, data=dataset)
# display results
summary(mod)
# regression model only reading
mod2 <- lm(math ~ reading, data=dataset)
# display results
summary(mod2)

lm(formula = math ~ reading + group, data = dataset)

Residuals:
    Min      1Q  Median      3Q     Max
-2.0430 -0.3655  0.1399  0.5040  1.3478

Coefficients:
            Estimate Std. Error t value Pr(>|t|)
(Intercept)   3.5242     1.6383   2.151   0.0685 .
reading       0.6405     0.2116   3.027   0.0192 *
group        -3.3400     1.0080  -3.314   0.0129 *

lm(formula = math ~ reading, data = dataset)

Residuals:
    Min      1Q  Median      3Q     Max
-3.5464 -0.8077 -0.0328  1.1261  2.2719

Coefficients:
            Estimate Std. Error t value Pr(>|t|)
(Intercept)  -0.7441     1.5176  -0.490  0.63706
reading       1.0908     0.2431   4.487  0.00204 **

# get means
descriptive <- dataset %>%
               group_by(group) %>%
               summarise_all(funs(mean(.),n()))
# insert predicted values for each model
dataset$math_full         <- predict(mod)
dataset$math_reading_only <- predict(mod2)
# get the difference between each prediction
dataset$difference <- abs(dataset$math_full - dataset$math_reading_only)

# A tibble: 2 x 11
  group math_mean reading_mean math_full_mean math_reading_only_mean difference_mean math_n
  <dbl>     <dbl>        <dbl>          <dbl>                  <dbl>           <dbl>  <int>
1     0       8.2         7.30            8.2               7.218976       0.9810243      5
2     1       2.9         4.24            2.9               3.881024       1.3343249      5
# ... with 4 more variables: reading_n <int>, math_full_n <int>, math_reading_only_n <int>,
#   difference_n <int>