First you can find [tex] \frac{31}{45} [/tex] of 90.
90 x [tex] \frac{31}{45} [/tex] = 62
Next, you have $62 left and spent [tex] \frac{1}{7} [/tex] of it. Meaning you have [tex] \frac{6}{7} [/tex] of 62 left.
62 x [tex] \frac{6}{7} [/tex] = about 53.14285
As your answer is not a whole number, you could round according to whatever guidelines your homework has given you (for example - rounding to the 2nd decimal place $53.14)