McDonald's, Burger King, and Wendy's used a simple trick to sell 100 million extra meals

The rise of the combo deal is driving sales across the fast-food industry.

McDonald's, Burger King, and Wendy's have sold more than 100 million additional combo meals since late 2015, reports the NPD Group.

On average, deals account for about one in four of customer visits to quick service restaurants. However, now an average of 35% of customers' visits to the fast-food chains have been to purchase the bundled promotion. Simultaneously, sales at all three fast-food burger chains are up since debuting the combo deals.

Apparently, bargain shopping - not dedication to any one chain - is driving much of the rise in customer traffic. According to NDP Group's Checkout Tracking data, many who purchased combo deals visited multiple chains to take advantage of all the promotions.

The bundled deal has been the hottest deal in the world of fast food in 2016.

Wendy's started bundled bargain trend, launching the "four for $4" promotion in October, selling a junior bacon cheeseburger, chicken nuggets, fries, and a drink for $4.

McDonald's followed in Wendy's footsteps with the McPick 2 menu, a bundled deal that that company said helped drive a 5.4% first-quarter increase in sales at US locations open more than a year. McDonald's began testing a breakfast version of the deal in March, and is planning on expanding the deal to include regional favorites starting May 24.

Burger King answered with a "five for $4" promotion, announced just days after McDonald's rolled out the McPick 2. In April, Burger King's parent company, Restaurant Brands International, reported that the chain's sales at locations open more than a year increased 4.4% in the US and Canada, thanks in part to the bundled deal as well as the successful launch of Grilled Dogs.

Now, even chains outside of the fast-food business are exploring the bundled bargain. Starbucks is currently offering a limited-time $8 Power Lunch deal, that combines a sandwich, salad, or bistro box; popcorn, pretzels, or chips; banana or fruit bar; and a bottle of Ethos water. Earlier in May, TGI Fridays debuted the Drink & Dine menu, which combines an entrée and beverage for $12.