If there are 1030 grains of sand on playa de sonrisa, how many of these grains of sand are submerged by the ocean at least once a day?

85 percent

Explanation:

(1) 40% of the grains of sand are submerged at least twice a day --> 0.4*1030 grains are submerged twice or more. Not sufficient,

(2) 55% of the grains of sand are not submerged exactly once a day. This imply that the remaining 45%, so 0.45*1030 grains, ARE submerged exactly once a day. Not sufficient.

(1)+(2) We have that 45% are submerged exactly once and 40% are submerged twice or more. Therefore, 45% + 40% = 85% are submerged at least once (once ore more). Sufficient.