hello gelene! we know that mervin received $90 from rachel and $50 from his mother. the reason why i only add the $50 is because the $90 is part of the $750 that the 2 siblings have.

so after mervin received all the money, he had 1/3 as much savings (refer to the model i drew). so in total, there are 4 units. we find the amount rachel has by finding out the amount of the three units and add $90 into the equation.

let me know if you need me to explain further ok! :)