If you sell 40 white melons every day; Watermelons and white melons are sold at 7: 5.

Watermelons are sold every day: 40÷5x7=56 (pieces).

And the difference between selling 50 pieces a day: 56-50=6 (pieces)

Sold: 36÷6=6 (days)

Watermelon in fruit shop: 50x6+36=336 (each)