Ratio of boys to girls is 3 to 4, the team has 70 members. how many are boys.

Working:
3:4 and 70 members
3 + 4 = 7
7/70 = 10
Then multiply 10, to each 'unit'. Assuming, 3:4 is boys to girls respectfully, the unit of boys represented by the ratio is '3'
Therefore,
10 × 3 = 30 [boys]
10 × 4 = 40 [girls]
There are 30 males, and 40 females in the team. 

Hope this helps :)

B, 0.4 repeats so it is rational and square root of 16 is 4 so that is rational as well.