Diophantus, a Greek mathematician who lived in the 3rd Century B. C., found a connection between triangular numbers (T) and square numbers (K) Find the square number when the triangular number is
a). 6
b). 21
c). 78
d). 276
e). 406
f). 820

its c or d   take a lucky guess my dude

let x and 400 - x be the two numbers.  

==> 0.80x + 0.85(400 - x) = 400 - 68 from the statement of the problem.  

< ==> 0.80x + 0.85(400 - x) = 400 - 68  

==> 340 - 0.05x = 332  

< ==> -0.05x = -8 ==> x = 160, the first number ==> 400 - x = 240, the second number.