SOLUTION
In 2020 black engineers under 30 is 70% of 500, which is
[tex]\frac{70}{100}\times500=350[/tex]
In 2021 500 members joined making a total of 1000 members.
The black engineers together are now 50% of 1000, which is 500
So the numbers of black engineers that joined in 2021 becomes
[tex]500-350=150[/tex]
Hence the answer is 150
