Simplify : $$\sqrt{[(12.1)^2-(8.1)^2]÷[(0.25)^2+(0.25)(19.95)]} $$

If $$ {0.7\over 1-6c}=-0.2$$ then what is the value of c.

If $$ {3\sqrt{{1-a}\over{a}}}+9={19-3\sqrt{{a}\over{1-a}}}$$, then what is the value of a?

Direction: what should come in place at question mark(?).

45% of 224×?% of 120=8104.32

In a group of 15 women, 7 have nose studs, 8 have ear rings and 3 have neither. How many of these have both nose studs and ear rings? 

1-[5-{2+(-5+6-2)2}] is equal to 

The value of   $${(75.8)^2-(55.8)^2\over20}$$

Direction: what should come in place at question mark(?).

$$ {4\over 7}×{2{4\over 5}÷{2\over 5}=?}$$