Answer: 33.33%

If suppose sp is 100% (assume)

Then gain % is 25%..

Then subtract it from sp we will get cp i.e 100-25=75%.

We know two basic formulas %gain=(gain/cp)*100 . or %loss=(loss/cp)*100.

So %gain=(25/75)*100=33.33

-Balue

Let SP be 100

x*10=(x-10)*(10*2)

10x =20x-200
20x-10x=200
10x=200
x=20

check it if any step is wrong

Let no of men  = x.
x men can do a piece of work in 10 days.
so, 1 man can do it in 10*x days.
Now (x-10) men do the work in 20 days.
so, 1 man ... in 20*(x-10).
so, 20(x-10) = 10x.
so, x = 20.
Ans is option (d).

A's age=9, B's age=38

Let present ages be 'a' and 'b'.
after 10 yrs A's age will be  a+10
and that of B will be b+10

age of B before 10 yrs  = b-10

so given condition is
a+10=2(b-10)

second condition is
a-b=9

solving these two equations we get
b=39

so after 10 yrs B's age will be 39+10=49