769=40a+(1.5a×5)

769 = 40a + (1.5a * 5)

(1.5 a * 5) = (7.5a)

769 = 40a + 7.5a

769 = 47.5a

769/47.5

$${\frac{{\mathtt{769}}}{{\mathtt{47.5}}}} = {\frac{{\mathtt{1\,538}}}{{\mathtt{95}}}} = {\mathtt{16.189\: \!473\: \!684\: \!210\: \!526\: \!3}}$$

16.189 = a

769=40a+(1.5a×5)

a=16.1894736842105263

But if you round it because it's a large number, this is what you get as an answer:

a=16.189