47. $4000 invested at 4%, $6000 invested at 5%, and $10,000 invested at 6%

49. Alex worked 2 hours, Becky worked 2.5 hours, and Courtney worked 1.5 hours.

51. 56 nickels, 225 dimes, and 19 quarters

53. 31 hours for normal daytime work, 14 hours at night, and 8 hours on a holiday

55. Cell A contains healthy tissue $(\text{because }x=0.21),$ cell B contains tumorous tissue $(\text{because }y=0.33),$ and cell C contains healthy ­tissue $(\text{because }z=0.19).$

57. Cells A, B, and C contain healthy tissue (because $x=0.28,y=0.23,$ and $z=0.21$).