Target question => what was Bill's average (arithmetic mean) score for all 4 tests?

Statement 1 => Bill's first 3 tests recovered an average score of 50
There is no information about the 4th test so overall average of the 4 test is unknown. Statement 1 is NOT SUFFICIENT

Statement 2 => Bill's last 2 tests received an average score of 50
There is no information about the1st and 2nd test so overall average of the test is unknown. Statement 2 is NOT SUFFICIENT

$$Combining\ both\ statements\ together\ =>\ $$
$$From\ statement\ 1\ =>\frac{test\ 1\ +\ test\ 2\ +\ test\ 3}{3}=50$$
$$From\ statement\ 2\ =>\frac{test\ 3\ +\ test\ 4}{2}=50$$
$$Scenarios\ that\ satisfy\ both\ statements\ include:$$
$$\frac{100+50+0+100}{4}=\frac{250}{4}\ Average\ =\ 62.5$$
$$or\ \frac{50+50+50+50}{4}=\frac{200}{4}\ Average\ =\ 50$$
$$or\ \frac{0+50+100+0}{4}=\frac{150}{4}\ Average\ =\ 37.5$$

Since the target question cannot be answered with certainty , both statements combined together ARE NOT SUFFICIENT

Answer = E