Question 1822
let his third exam result be x, so that we need the average of all 3 to be equal to 90%. This is the English version, now for the maths version ...

{{{(91+87+x)/3 = 90}}}
{{{(91+87+x) = 270}}}

therefore {{{x = 270 - 91 - 87}}}

x = 92%

cheers
Jon.