The sum of two numbers is 21 .two-thirds of one number is added to three-quarters of the other number to give 15 .find the two numbers,showing clearly your workings.
simple translation from English to Math (after all Math is just a language)
let 1st number be x
let 2nd number be y
x + y = 21
(2/3)x + (3/4)y = 15
multiply each term by 12
8x + 9y = 180
I would use substitution,
from the 1st equation ----> y = 21-x
sub that into the 2nd
go for it, just a few more steps