recipe requires 15 mins to cook . you have 7 mins hourglass and a 11 mins hourglass with you . how will you measure 15 mins using them ?
correct answer : a put up both hour glasses . time starts when 7 min hourglass turns empty . and 11 min hourglass will run for 4 min . now flip 11 min hour glass upside down as soon as it runs empty . it will be empty in another 11 mins . and time is up . so 4 + 11 = 15 mins
add(11, subtract(11, 7))