The first thing is to identify the variable (x) of the equation and they will be the number of days that Leilah will take to read her flashcards. Now if you need to study 600 out of 2400, it is because you have read 1800. Therefore we have to:
Leilah has not yet studied 600 of her 2400 flashcards. She studies 404040 new cards each day. 2400-40x=600
Stetson rents studio space for $600a month for music lessons. He charges his students $40 per hour and earned a profit of $2400 this month.40x-600=2400
A kit contains 600 letter tiles and 40 number tiles. Each tile has the same mass, and the kit has a total mass of 2400g (600+40)x=2400
