The way to solve this type of problem is to choose just one unknown and then calculate other unknowns in terms of the unknown you’ve selected. So in this problem our unknown could be the number of Scott’s messages, so we let S=number of his messages. The number of Raina’s messages must be 7 fewer than Scott’s so we have S-7 for Raina. Abdul has 4 times as many as Scott, so that’s 4S.

And we know how many messages there are so we add these together S+S-7+4S=143. That gives us 6S-7=143. Add 7 to each side: 6S=150 so S=150/6=25. Scott has 25 messages.

Raina has 25-7=18 and Abdul has 4×25=100.