mathcounts problems 81 through 85

81. If (x+y)^2=x^2+y^2, what is the value of xy?

82. Kiera and Aubrey were the only candidates for SGA President. Kiera received 44% of the 7th graders’ votes, and Aubrey received 42% of the 8th graders’ votes. The student body consists of 325 7th graders and 350 8th graders, and every student voted for one of these two candidates. What percent of the students’ votes did the winner receive? Express your answer to the nearest whole number.

83. Enoch has a photo that is 8 inches high and 6 inches wide. He wants to enlarge it so that it can fit into a frame that is 10 inches by 7 inches. How much will he need to crop from the width to exactly fill the entire frame? Express your answer as a decimal to the nearest tenth.

84. At 1:00 p.m. an airplane left the local airport and flew due east at 300 mi/h. At 3:00 p.m. a second plane left the same airport and flew due north at 400 mi/h. Assuming the curvature of the earth is negligible, how many miles apart are the planes at 8:00 p.m.?

85. If the mean of the integers 7, 3, 11, 13, 5 and x is 4 more than the mode of the six integers, what is the value of x?