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 ?
"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 years answer is a"
add(add(multiply(const_2, 10), 10), 9)