Today’s Programming Praxis problem is an easy one. We’re supposed to give the prices of four items, that both sum up and multiply to $7.11. Let’s get started.

While we could just do a brute force test on all possible combinations, this would take rather long. So in order to speed things up we only check numbers that are a proper divisor of $7.11.

divs :: [Int] divs = [x | x <- [1..711], mod (711 * 10^6) x == 0]

Once we have the divisors, solving the problem becomes a fairly trivial list comprehension (the <= bits are another optimization to eliminate some identical combinations).

main :: IO () main = print [(a,b,c,d) | a <- divs, b <- divs, b <= a, c <- divs, c <= b, d <- divs, d <= c, a + b + c + d == 711, a * b * c * d == 711 * 10^6]

If we run this, we find there is only one combination that satisfies both requirements.