in 20 years , a will be twice as old as b was 20 years ago . if a is now 10 years older than b , the present age of b is ?
"let b ' s present age = x years then a ' s present age = x + 10 years x + 10 + 20 = 2 ( x - 20 ) x + 30 = 2 x - 40 x = 70 years answer is e"
add(add(multiply(const_2, 20), 20), 10)