Devi and her brother had the same amount of money. after devi spent 2/5 of her money and her brother spent 3/10 of his money, they had $78 left altogether. how much did they spend altogether?

To answer this question you have to create a system of equations. The first equation will be that Devi's money (x) equals her brother's money (y), or x = y. The next equation would be that (3/5)x + (7/10)y = 78. You than can substitute x in for y because x = y. The equation would know be (3/5)x + (7/10)x = 78. You then combine the like terms to create an equation of (13/10)x = 78. Then, multiply both sides by 10 / 13 in order to isolate x. This will create the equation x = 60. This means that Devi and her brother each had 60 dollars. You then find out how much they spent and add it together. You can do so with the equation (2/5)x + (3/10)y = z, with z being total money spent. You substitute 60 in for x and for y then solve. When you solve you see that 24 + 18 = z, or that z equals 42. In other words, they spent 42 dollars altogether.