in 10 years , a will be twice as old as b was 10 years ago . if a is now 9 years older than b , the present age of b is :
"explanation : let b ' s present age = x years . then , a ' s present age = ( x + 9 ) years . ( x + 9 ) + 10 = 2 ( x - 10 ) = > x + 19 = 2 x - 20 = > x = 39 . answer : c"
add(add(multiply(const_2, 10), 10), 9)